diff --git a/docs/changelog/129990.yaml b/docs/changelog/129990.yaml
new file mode 100644
index 0000000000000..aa5ca924f2932
--- /dev/null
+++ b/docs/changelog/129990.yaml
@@ -0,0 +1,5 @@
+pr: 129990
+summary: Make forecast write load accurate when shard numbers change
+area: Allocation
+type: bug
+issues: []
diff --git a/server/src/main/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingService.java b/server/src/main/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingService.java
index 61eb2867790a2..334e0c7564f7c 100644
--- a/server/src/main/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingService.java
+++ b/server/src/main/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingService.java
@@ -362,7 +362,7 @@ public String toString() {
      * <p>If the recommendation is to INCREASE/DECREASE shards the reported cooldown period will be TimeValue.ZERO.
      * If the auto sharding service thinks the number of shards must be changed but it can't recommend a change due to the cooldown
      * period not lapsing, the result will be of type {@link AutoShardingType#COOLDOWN_PREVENTED_INCREASE} or
-     * {@link AutoShardingType#COOLDOWN_PREVENTED_INCREASE} with the remaining cooldown configured and the number of shards that should
+     * {@link AutoShardingType#COOLDOWN_PREVENTED_DECREASE} with the remaining cooldown configured and the number of shards that should
      * be configured for the data stream once the remaining cooldown lapses as the target number of shards.
      *
      * <p>The NOT_APPLICABLE type result will report a cooldown period of TimeValue.MAX_VALUE.
diff --git a/x-pack/plugin/write-load-forecaster/src/main/java/org/elasticsearch/xpack/writeloadforecaster/LicensedWriteLoadForecaster.java b/x-pack/plugin/write-load-forecaster/src/main/java/org/elasticsearch/xpack/writeloadforecaster/LicensedWriteLoadForecaster.java
index b9b900836c6fc..db68694af4143 100644
--- a/x-pack/plugin/write-load-forecaster/src/main/java/org/elasticsearch/xpack/writeloadforecaster/LicensedWriteLoadForecaster.java
+++ b/x-pack/plugin/write-load-forecaster/src/main/java/org/elasticsearch/xpack/writeloadforecaster/LicensedWriteLoadForecaster.java
@@ -108,7 +108,12 @@ public ProjectMetadata.Builder withWriteLoadForecastForWriteIndex(String dataStr
         }
 
         final IndexMetadata writeIndex = metadata.getSafe(dataStream.getWriteIndex());
-        metadata.put(IndexMetadata.builder(writeIndex).indexWriteLoadForecast(forecastIndexWriteLoad.getAsDouble()).build(), false);
+        metadata.put(
+            IndexMetadata.builder(writeIndex)
+                .indexWriteLoadForecast(forecastIndexWriteLoad.getAsDouble() / writeIndex.getNumberOfShards())
+                .build(),
+            false
+        );
 
         return metadata;
     }
@@ -129,11 +134,20 @@ private static void clearPreviousForecast(DataStream dataStream, ProjectMetadata
         }
     }
 
+    /**
+     * This calculates the weighted average total write-load for all recent indices.
+     *
+     * @param indicesWriteLoadWithinMaxAgeRange The indices considered "recent"
+     * @return The weighted average total write-load. To get the per-shard write load, this number must be divided by the number of shards
+     */
     // Visible for testing
     static OptionalDouble forecastIndexWriteLoad(List<IndexWriteLoad> indicesWriteLoadWithinMaxAgeRange) {
-        double totalWeightedWriteLoad = 0;
-        long totalShardUptime = 0;
+        double allIndicesWriteLoad = 0;
+        long allIndicesUptime = 0;
         for (IndexWriteLoad writeLoad : indicesWriteLoadWithinMaxAgeRange) {
+            double totalShardWriteLoad = 0;
+            long totalShardUptimeInMillis = 0;
+            long maxShardUptimeInMillis = 0;
             for (int shardId = 0; shardId < writeLoad.numberOfShards(); shardId++) {
                 final OptionalDouble writeLoadForShard = writeLoad.getWriteLoadForShard(shardId);
                 final OptionalLong uptimeInMillisForShard = writeLoad.getUptimeInMillisForShard(shardId);
@@ -141,13 +155,27 @@ static OptionalDouble forecastIndexWriteLoad(List<IndexWriteLoad> indicesWriteLo
                     assert uptimeInMillisForShard.isPresent();
                     double shardWriteLoad = writeLoadForShard.getAsDouble();
                     long shardUptimeInMillis = uptimeInMillisForShard.getAsLong();
-                    totalWeightedWriteLoad += shardWriteLoad * shardUptimeInMillis;
-                    totalShardUptime += shardUptimeInMillis;
+                    totalShardWriteLoad += shardWriteLoad * shardUptimeInMillis;
+                    totalShardUptimeInMillis += shardUptimeInMillis;
+                    maxShardUptimeInMillis = Math.max(maxShardUptimeInMillis, shardUptimeInMillis);
                 }
             }
+            double weightedAverageShardWriteLoad = totalShardWriteLoad / totalShardUptimeInMillis;
+            double totalIndexWriteLoad = weightedAverageShardWriteLoad * writeLoad.numberOfShards();
+            // We need to weight the contribution from each index somehow, but we only know
+            // the write-load from the final allocation of each shard at rollover time. It's
+            // possible the index is much older than any of those shards, but we don't have
+            // any write-load data beyond their lifetime.
+            // To avoid making assumptions about periods for which we have no data, we'll weight
+            // each index's contribution to the forecast by the maximum shard uptime observed in
+            // that index. It should be safe to extrapolate our weighted average out to the
+            // maximum uptime observed, based on the assumption that write-load is roughly
+            // evenly distributed across shards of a datastream index.
+            allIndicesWriteLoad += totalIndexWriteLoad * maxShardUptimeInMillis;
+            allIndicesUptime += maxShardUptimeInMillis;
         }
 
-        return totalShardUptime == 0 ? OptionalDouble.empty() : OptionalDouble.of(totalWeightedWriteLoad / totalShardUptime);
+        return allIndicesUptime == 0 ? OptionalDouble.empty() : OptionalDouble.of(allIndicesWriteLoad / allIndicesUptime);
     }
 
     @Override
diff --git a/x-pack/plugin/write-load-forecaster/src/test/java/org/elasticsearch/xpack/writeloadforecaster/LicensedWriteLoadForecasterTests.java b/x-pack/plugin/write-load-forecaster/src/test/java/org/elasticsearch/xpack/writeloadforecaster/LicensedWriteLoadForecasterTests.java
index d3db529f3cda7..3062e9a82b2ac 100644
--- a/x-pack/plugin/write-load-forecaster/src/test/java/org/elasticsearch/xpack/writeloadforecaster/LicensedWriteLoadForecasterTests.java
+++ b/x-pack/plugin/write-load-forecaster/src/test/java/org/elasticsearch/xpack/writeloadforecaster/LicensedWriteLoadForecasterTests.java
@@ -9,6 +9,7 @@
 
 import org.apache.logging.log4j.Level;
 import org.apache.logging.log4j.core.LogEvent;
+import org.apache.lucene.util.hnsw.IntToIntFunction;
 import org.elasticsearch.cluster.metadata.DataStream;
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.cluster.metadata.IndexMetadataStats;
@@ -24,16 +25,19 @@
 import org.elasticsearch.test.MockLog;
 import org.elasticsearch.threadpool.TestThreadPool;
 import org.elasticsearch.threadpool.ThreadPool;
+import org.hamcrest.Matcher;
 import org.junit.After;
 import org.junit.Before;
 
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.OptionalDouble;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.stream.Collectors;
 
 import static org.elasticsearch.xpack.writeloadforecaster.LicensedWriteLoadForecaster.forecastIndexWriteLoad;
 import static org.hamcrest.Matchers.closeTo;
@@ -42,6 +46,7 @@
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.lessThan;
 
 public class LicensedWriteLoadForecasterTests extends ESTestCase {
     ThreadPool threadPool;
@@ -67,33 +72,15 @@ public void testWriteLoadForecastIsAddedToWriteIndex() {
 
         writeLoadForecaster.refreshLicense();
 
-        final ProjectMetadata.Builder metadataBuilder = ProjectMetadata.builder(randomProjectIdOrDefault());
         final String dataStreamName = "logs-es";
         final int numberOfBackingIndices = 10;
-        final int numberOfShards = randomIntBetween(1, 5);
-        final List<Index> backingIndices = new ArrayList<>();
-        for (int i = 0; i < numberOfBackingIndices; i++) {
-            final IndexMetadata indexMetadata = createIndexMetadata(
-                DataStream.getDefaultBackingIndexName(dataStreamName, i),
-                numberOfShards,
-                randomIndexWriteLoad(numberOfShards),
-                System.currentTimeMillis() - (maxIndexAge.millis() / 2)
-            );
-            backingIndices.add(indexMetadata.getIndex());
-            metadataBuilder.put(indexMetadata, false);
-        }
-
-        final IndexMetadata writeIndexMetadata = createIndexMetadata(
-            DataStream.getDefaultBackingIndexName(dataStreamName, numberOfBackingIndices),
-            numberOfShards,
-            null,
-            System.currentTimeMillis()
+        final ProjectMetadata.Builder metadataBuilder = createMetadataBuilderWithDataStream(
+            dataStreamName,
+            numberOfBackingIndices,
+            randomIntBetween(1, 5),
+            maxIndexAge
         );
-        backingIndices.add(writeIndexMetadata.getIndex());
-        metadataBuilder.put(writeIndexMetadata, false);
-
-        final DataStream dataStream = createDataStream(dataStreamName, backingIndices);
-        metadataBuilder.put(dataStream);
+        final DataStream dataStream = metadataBuilder.dataStream(dataStreamName);
 
         final ProjectMetadata.Builder updatedMetadataBuilder = writeLoadForecaster.withWriteLoadForecastForWriteIndex(
             dataStream.getName(),
@@ -253,7 +240,7 @@ public void testWriteLoadForecast() {
                 )
             );
             assertThat(writeLoadForecast.isPresent(), is(true));
-            assertThat(writeLoadForecast.getAsDouble(), is(equalTo(14.4)));
+            assertThat(writeLoadForecast.getAsDouble(), is(equalTo(72.0)));
         }
 
         {
@@ -264,14 +251,14 @@ public void testWriteLoadForecast() {
                         .withShardWriteLoad(1, 24, 999, 999, 5)
                         .withShardWriteLoad(2, 24, 999, 999, 5)
                         .withShardWriteLoad(3, 24, 999, 999, 5)
-                        .withShardWriteLoad(4, 24, 999, 999, 4)
+                        .withShardWriteLoad(4, 24, 999, 999, 5)
                         .build(),
                     // Since this shard uptime is really low, it doesn't add much to the avg
                     IndexWriteLoad.builder(1).withShardWriteLoad(0, 120, 999, 999, 1).build()
                 )
             );
             assertThat(writeLoadForecast.isPresent(), is(true));
-            assertThat(writeLoadForecast.getAsDouble(), is(equalTo(15.36)));
+            assertThat(writeLoadForecast.getAsDouble(), is(closeTo(72.59, 0.01)));
         }
 
         {
@@ -283,7 +270,7 @@ public void testWriteLoadForecast() {
                 )
             );
             assertThat(writeLoadForecast.isPresent(), is(true));
-            assertThat(writeLoadForecast.getAsDouble(), is(equalTo(12.0)));
+            assertThat(writeLoadForecast.getAsDouble(), is(equalTo(16.0)));
         }
 
         {
@@ -302,7 +289,7 @@ public void testWriteLoadForecast() {
                 )
             );
             assertThat(writeLoadForecast.isPresent(), is(true));
-            assertThat(writeLoadForecast.getAsDouble(), is(closeTo(15.83, 0.01)));
+            assertThat(writeLoadForecast.getAsDouble(), is(closeTo(31.66, 0.01)));
         }
     }
 
@@ -404,4 +391,163 @@ public boolean innerMatch(LogEvent event) {
             );
         }, LicensedWriteLoadForecaster.class, collectingLoggingAssertion);
     }
+
+    public void testShardIncreaseDoesNotIncreaseTotalLoad() {
+        testShardChangeDoesNotChangeTotalForecastLoad(ShardCountChange.INCREASE);
+    }
+
+    public void testShardDecreaseDoesNotDecreaseTotalLoad() {
+        testShardChangeDoesNotChangeTotalForecastLoad(ShardCountChange.DECREASE);
+    }
+
+    private void testShardChangeDoesNotChangeTotalForecastLoad(ShardCountChange shardCountChange) {
+        final TimeValue maxIndexAge = TimeValue.timeValueDays(7);
+        final AtomicBoolean hasValidLicense = new AtomicBoolean(true);
+        final AtomicInteger licenseCheckCount = new AtomicInteger();
+        final WriteLoadForecaster writeLoadForecaster = new LicensedWriteLoadForecaster(() -> {
+            licenseCheckCount.incrementAndGet();
+            return hasValidLicense.get();
+        }, threadPool, maxIndexAge);
+        writeLoadForecaster.refreshLicense();
+
+        final String dataStreamName = randomIdentifier();
+        final ProjectMetadata.Builder originalMetadata = writeLoadForecaster.withWriteLoadForecastForWriteIndex(
+            dataStreamName,
+            createMetadataBuilderWithDataStream(dataStreamName, randomIntBetween(5, 15), shardCountChange.originalShardCount(), maxIndexAge)
+        );
+
+        // Generate the same data stream, but with a different number of shards in the write index
+        final ProjectMetadata.Builder changedShardCountMetadata = writeLoadForecaster.withWriteLoadForecastForWriteIndex(
+            dataStreamName,
+            updateWriteIndexShardCount(dataStreamName, originalMetadata, shardCountChange)
+        );
+
+        IndexMetadata originalWriteIndexMetadata = originalMetadata.getSafe(originalMetadata.dataStream(dataStreamName).getWriteIndex());
+        IndexMetadata changedShardCountWriteIndexMetadata = changedShardCountMetadata.getSafe(
+            changedShardCountMetadata.dataStream(dataStreamName).getWriteIndex()
+        );
+
+        // The shard count changed
+        assertThat(
+            changedShardCountWriteIndexMetadata.getNumberOfShards(),
+            shardCountChange.expectedChangeFromOriginal(originalWriteIndexMetadata.getNumberOfShards())
+        );
+        // But the total write-load did not
+        assertThat(
+            changedShardCountWriteIndexMetadata.getNumberOfShards() * writeLoadForecaster.getForecastedWriteLoad(
+                changedShardCountWriteIndexMetadata
+            ).getAsDouble(),
+            closeTo(
+                originalWriteIndexMetadata.getNumberOfShards() * writeLoadForecaster.getForecastedWriteLoad(originalWriteIndexMetadata)
+                    .getAsDouble(),
+                0.01
+            )
+        );
+    }
+
+    public enum ShardCountChange implements IntToIntFunction {
+        INCREASE(1, 15) {
+            @Override
+            public int apply(int originalShardCount) {
+                return randomIntBetween(originalShardCount + 1, originalShardCount * 3);
+            }
+
+            public Matcher<Integer> expectedChangeFromOriginal(int originalShardCount) {
+                return greaterThan(originalShardCount);
+            }
+        },
+        DECREASE(10, 30) {
+            @Override
+            public int apply(int originalShardCount) {
+                return randomIntBetween(1, originalShardCount - 1);
+            }
+
+            public Matcher<Integer> expectedChangeFromOriginal(int originalShardCount) {
+                return lessThan(originalShardCount);
+            }
+        };
+
+        private final int originalMinimumShardCount;
+        private final int originalMaximumShardCount;
+
+        ShardCountChange(int originalMinimumShardCount, int originalMaximumShardCount) {
+            this.originalMinimumShardCount = originalMinimumShardCount;
+            this.originalMaximumShardCount = originalMaximumShardCount;
+        }
+
+        public int originalShardCount() {
+            return randomIntBetween(originalMinimumShardCount, originalMaximumShardCount);
+        }
+
+        abstract Matcher<Integer> expectedChangeFromOriginal(int originalShardCount);
+    }
+
+    private ProjectMetadata.Builder updateWriteIndexShardCount(
+        String dataStreamName,
+        ProjectMetadata.Builder originalMetadata,
+        ShardCountChange shardCountChange
+    ) {
+        final ProjectMetadata.Builder updatedShardCountMetadata = ProjectMetadata.builder(originalMetadata.getId());
+
+        final DataStream originalDataStream = originalMetadata.dataStream(dataStreamName);
+        final Index existingWriteIndex = Objects.requireNonNull(originalDataStream.getWriteIndex());
+        final IndexMetadata originalWriteIndexMetadata = originalMetadata.getSafe(existingWriteIndex);
+
+        // Copy all non-write indices over unchanged
+        final List<IndexMetadata> backingIndexMetadatas = originalDataStream.getIndices()
+            .stream()
+            .filter(index -> index != existingWriteIndex)
+            .map(originalMetadata::getSafe)
+            .collect(Collectors.toList());
+
+        // Create a new write index with an updated shard count
+        final IndexMetadata writeIndexMetadata = createIndexMetadata(
+            DataStream.getDefaultBackingIndexName(dataStreamName, backingIndexMetadatas.size()),
+            shardCountChange.apply(originalWriteIndexMetadata.getNumberOfShards()),
+            null,
+            System.currentTimeMillis()
+        );
+        backingIndexMetadatas.add(writeIndexMetadata);
+        backingIndexMetadatas.forEach(indexMetadata -> updatedShardCountMetadata.put(indexMetadata, false));
+
+        final DataStream dataStream = createDataStream(
+            dataStreamName,
+            backingIndexMetadatas.stream().map(IndexMetadata::getIndex).toList()
+        );
+        updatedShardCountMetadata.put(dataStream);
+        return updatedShardCountMetadata;
+    }
+
+    private ProjectMetadata.Builder createMetadataBuilderWithDataStream(
+        String dataStreamName,
+        int numberOfBackingIndices,
+        int numberOfShards,
+        TimeValue maxIndexAge
+    ) {
+        final ProjectMetadata.Builder metadataBuilder = ProjectMetadata.builder(randomProjectIdOrDefault());
+        final List<Index> backingIndices = new ArrayList<>();
+        for (int i = 0; i < numberOfBackingIndices; i++) {
+            final IndexMetadata indexMetadata = createIndexMetadata(
+                DataStream.getDefaultBackingIndexName(dataStreamName, i),
+                numberOfShards,
+                randomIndexWriteLoad(numberOfShards),
+                System.currentTimeMillis() - (maxIndexAge.millis() / 2)
+            );
+            backingIndices.add(indexMetadata.getIndex());
+            metadataBuilder.put(indexMetadata, false);
+        }
+
+        final IndexMetadata writeIndexMetadata = createIndexMetadata(
+            DataStream.getDefaultBackingIndexName(dataStreamName, numberOfBackingIndices),
+            numberOfShards,
+            null,
+            System.currentTimeMillis()
+        );
+        backingIndices.add(writeIndexMetadata.getIndex());
+        metadataBuilder.put(writeIndexMetadata, false);
+
+        final DataStream dataStream = createDataStream(dataStreamName, backingIndices);
+        metadataBuilder.put(dataStream);
+        return metadataBuilder;
+    }
 }