diff --git a/block.go b/block.go
index fc9ed27..c73706d 100644
--- a/block.go
+++ b/block.go
@@ -1399,6 +1399,7 @@ func (c *Chain) RecentBlockStats(samples int) (hashrate float64, avgBlockTime fl
 		return 0, 0
 	}
 	var totalTime int64
+	var totalWork uint64
 	var count int
 	for h := height; h > 0 && count < samples; h-- {
 		block := c.GetBlockByHeight(h)
@@ -1409,6 +1410,7 @@ func (c *Chain) RecentBlockStats(samples int) (hashrate float64, avgBlockTime fl
 		dt := block.Header.Timestamp - prev.Header.Timestamp
 		if dt > 0 {
 			totalTime += dt
+			totalWork += block.Header.Difficulty
 			count++
 		}
 	}
@@ -1416,7 +1418,10 @@ func (c *Chain) RecentBlockStats(samples int) (hashrate float64, avgBlockTime fl
 		return 0, 0
 	}
 	avgBlockTime = float64(totalTime) / float64(count)
-	hashrate = float64(c.NextDifficulty()) / avgBlockTime
+	// Each sampled interval ended with `block`, so its observed work is that
+	// block's difficulty. Using the next-block difficulty here biases the
+	// estimate whenever difficulty changes during the sample window.
+	hashrate = float64(totalWork) / float64(totalTime)
 	return hashrate, avgBlockTime
 }
 
diff --git a/block_stats_test.go b/block_stats_test.go
new file mode 100644
index 0000000..fbb0a48
--- /dev/null
+++ b/block_stats_test.go
@@ -0,0 +1,48 @@
+package main
+
+import (
+	"math"
+	"testing"
+)
+
+func TestRecentBlockStatsUsesObservedIntervalWork(t *testing.T) {
+	chain, storage, cleanup := mustCreateTestChain(t)
+	defer cleanup()
+	mustAddGenesisBlock(t, chain)
+
+	genesis := chain.GetBlockByHeight(0)
+	prev := genesis
+	timestamp := genesis.Header.Timestamp
+	var totalWork uint64
+	for height, difficulty := range []uint64{100, 300, 500} {
+		timestamp += int64((height + 1) * 10)
+		block := &Block{Header: BlockHeader{
+			Version:    1,
+			Height:     uint64(height + 1),
+			PrevHash:   prev.Hash(),
+			Timestamp:  timestamp,
+			Difficulty: difficulty,
+		}}
+		if err := storage.SaveBlock(block); err != nil {
+			t.Fatalf("save block %d: %v", height+1, err)
+		}
+		if err := storage.SetMainChainBlock(uint64(height+1), block.Hash()); err != nil {
+			t.Fatalf("index block %d: %v", height+1, err)
+		}
+		prev = block
+		totalWork += difficulty
+	}
+
+	chain.mu.Lock()
+	chain.height = 3
+	chain.bestHash = prev.Hash()
+	chain.mu.Unlock()
+
+	hashrate, avgBlockTime := chain.RecentBlockStats(3)
+	if want := 20.0; math.Abs(avgBlockTime-want) > 1e-9 {
+		t.Fatalf("average block time = %v, want %v", avgBlockTime, want)
+	}
+	if want := float64(totalWork) / 60.0; math.Abs(hashrate-want) > 1e-9 {
+		t.Fatalf("network hashrate = %v, want observed work/time %v", hashrate, want)
+	}
+}