diff --git a/docs/user/monitoring/kibana-alerts.asciidoc b/docs/user/monitoring/kibana-alerts.asciidoc
index bbc9c41c6ca5a..2944921edd2ee 100644
--- a/docs/user/monitoring/kibana-alerts.asciidoc
+++ b/docs/user/monitoring/kibana-alerts.asciidoc
@@ -81,8 +81,8 @@ by running checks on a schedule time of 1 minute with a re-notify interval of 6
 [[kibana-alerts-large-shard-size]]
 == Large shard size
 
-This alert is triggered if a large (primary) shard size is found on any of the 
-specified index patterns. The trigger condition is met if an index's shard size is 
+This alert is triggered if a large average shard size (across associated primaries) is found on any of the 
+specified index patterns. The trigger condition is met if an index's average shard size is 
 55gb or higher in the last 5 minutes. The alert is grouped across all indices that match 
 the default pattern of `*` by running checks on a schedule time of 1 minute with a re-notify 
 interval of 12 hours.
diff --git a/x-pack/plugins/monitoring/common/constants.ts b/x-pack/plugins/monitoring/common/constants.ts
index bf6e32af0dc39..cd3e28debb7d5 100644
--- a/x-pack/plugins/monitoring/common/constants.ts
+++ b/x-pack/plugins/monitoring/common/constants.ts
@@ -460,7 +460,7 @@ export const ALERT_DETAILS = {
     paramDetails: {
       threshold: {
         label: i18n.translate('xpack.monitoring.alerts.shardSize.paramDetails.threshold.label', {
-          defaultMessage: `Notify when a shard exceeds this size`,
+          defaultMessage: `Notify when average shard size exceeds this value`,
         }),
         type: AlertParamType.Number,
         append: 'GB',
@@ -477,7 +477,7 @@ export const ALERT_DETAILS = {
       defaultMessage: 'Shard size',
     }),
     description: i18n.translate('xpack.monitoring.alerts.shardSize.description', {
-      defaultMessage: 'Alert if an index (primary) shard is oversize.',
+      defaultMessage: 'Alert if the average shard size is larger than the configured threshold.',
     }),
   },
 };
diff --git a/x-pack/plugins/monitoring/common/types/es.ts b/x-pack/plugins/monitoring/common/types/es.ts
index 9dce32211f4b1..38a7e7859272c 100644
--- a/x-pack/plugins/monitoring/common/types/es.ts
+++ b/x-pack/plugins/monitoring/common/types/es.ts
@@ -100,6 +100,9 @@ export interface ElasticsearchNodeStats {
 
 export interface ElasticsearchIndexStats {
   index?: string;
+  shards: {
+    primaries: number;
+  };
   primaries?: {
     docs?: {
       count?: number;
diff --git a/x-pack/plugins/monitoring/server/alerts/large_shard_size_alert.ts b/x-pack/plugins/monitoring/server/alerts/large_shard_size_alert.ts
index 2c9e5a04e37e4..db318d7962beb 100644
--- a/x-pack/plugins/monitoring/server/alerts/large_shard_size_alert.ts
+++ b/x-pack/plugins/monitoring/server/alerts/large_shard_size_alert.ts
@@ -49,7 +49,7 @@ export class LargeShardSizeAlert extends BaseAlert {
           description: i18n.translate(
             'xpack.monitoring.alerts.shardSize.actionVariables.shardIndex',
             {
-              defaultMessage: 'List of indices which are experiencing large shard size.',
+              defaultMessage: 'List of indices which are experiencing large average shard size.',
             }
           ),
         },
@@ -100,7 +100,7 @@ export class LargeShardSizeAlert extends BaseAlert {
     const { shardIndex, shardSize } = item.meta as IndexShardSizeUIMeta;
     return {
       text: i18n.translate('xpack.monitoring.alerts.shardSize.ui.firingMessage', {
-        defaultMessage: `The following index: #start_link{shardIndex}#end_link has a large shard size of: {shardSize}GB at #absolute`,
+        defaultMessage: `The following index: #start_link{shardIndex}#end_link has a large average shard size of: {shardSize}GB at #absolute`,
         values: {
           shardIndex,
           shardSize,
diff --git a/x-pack/plugins/monitoring/server/lib/alerts/fetch_index_shard_size.ts b/x-pack/plugins/monitoring/server/lib/alerts/fetch_index_shard_size.ts
index f51e1cde47f8d..c3e9f08c3b949 100644
--- a/x-pack/plugins/monitoring/server/lib/alerts/fetch_index_shard_size.ts
+++ b/x-pack/plugins/monitoring/server/lib/alerts/fetch_index_shard_size.ts
@@ -69,13 +69,6 @@ export async function fetchIndexShardSize(
           },
           aggs: {
             over_threshold: {
-              filter: {
-                range: {
-                  'index_stats.primaries.store.size_in_bytes': {
-                    gt: threshold * gbMultiplier,
-                  },
-                },
-              },
               aggs: {
                 index: {
                   terms: {
@@ -96,6 +89,7 @@ export async function fetchIndexShardSize(
                         _source: {
                           includes: [
                             '_index',
+                            'index_stats.shards.primaries',
                             'index_stats.primaries.store.size_in_bytes',
                             'source_node.name',
                             'source_node.uuid',
@@ -123,7 +117,7 @@ export async function fetchIndexShardSize(
   if (!clusterBuckets.length) {
     return stats;
   }
-
+  const thresholdBytes = threshold * gbMultiplier;
   for (const clusterBucket of clusterBuckets) {
     const indexBuckets = clusterBucket.over_threshold.index.buckets;
     const clusterUuid = clusterBucket.key;
@@ -143,9 +137,25 @@ export async function fetchIndexShardSize(
         _source: { source_node: sourceNode, index_stats: indexStats },
       } = topHit;
 
-      const { size_in_bytes: shardSizeBytes } = indexStats?.primaries?.store!;
+      if (!indexStats || !indexStats.primaries) {
+        continue;
+      }
+
+      const { primaries: totalPrimaryShards } = indexStats.shards;
+      const { size_in_bytes: primaryShardSizeBytes = 0 } = indexStats.primaries.store || {};
+      if (!primaryShardSizeBytes || !totalPrimaryShards) {
+        continue;
+      }
+      /**
+       * We can only calculate the average primary shard size at this point, since we don't have
+       * data (in .monitoring-es* indices) to give us individual shards. This might change in the future
+       */
       const { name: nodeName, uuid: nodeId } = sourceNode;
-      const shardSize = +(shardSizeBytes! / gbMultiplier).toFixed(2);
+      const avgShardSize = primaryShardSizeBytes / totalPrimaryShards;
+      if (avgShardSize < thresholdBytes) {
+        continue;
+      }
+      const shardSize = +(avgShardSize / gbMultiplier).toFixed(2);
       stats.push({
         shardIndex,
         shardSize,