diff --git a/whisper/whisperv6/envelope.go b/whisper/whisperv6/envelope.go
index c42d1fa8a..5d9d35081 100644
--- a/whisper/whisperv6/envelope.go
+++ b/whisper/whisperv6/envelope.go
@@ -27,7 +27,6 @@ import (
 	"time"
 
 	"github.com/ethereum/go-ethereum/common"
-	"github.com/ethereum/go-ethereum/common/math"
 	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/crypto/ecies"
 	"github.com/ethereum/go-ethereum/rlp"
@@ -82,7 +81,7 @@ func (e *Envelope) Seal(options *MessageParams) error {
 		return nil
 	}
 
-	var target, bestBit int
+	var target, bestLeadingZeros int
 	if options.PoW < 0 {
 		// target is not set - the function should run for a period
 		// of time specified in WorkTime param. Since we can predict
@@ -101,10 +100,10 @@ func (e *Envelope) Seal(options *MessageParams) error {
 		for i := 0; i < 1024; i++ {
 			binary.BigEndian.PutUint64(buf[56:], nonce)
 			d := new(big.Int).SetBytes(crypto.Keccak256(buf))
-			firstBit := math.FirstBitSet(d)
-			if firstBit > bestBit {
-				e.Nonce, bestBit = nonce, firstBit
-				if target > 0 && bestBit >= target {
+			leadingZeros := 256 - d.BitLen()
+			if leadingZeros > bestLeadingZeros {
+				e.Nonce, bestLeadingZeros = nonce, leadingZeros
+				if target > 0 && bestLeadingZeros >= target {
 					return nil
 				}
 			}
@@ -112,7 +111,7 @@ func (e *Envelope) Seal(options *MessageParams) error {
 		}
 	}
 
-	if target > 0 && bestBit < target {
+	if target > 0 && bestLeadingZeros < target {
 		return fmt.Errorf("failed to reach the PoW target, specified pow time (%d seconds) was insufficient", options.WorkTime)
 	}
 
@@ -130,13 +129,14 @@ func (e *Envelope) PoW() float64 {
 
 func (e *Envelope) calculatePoW(diff uint32) {
 	buf := make([]byte, 64)
-	h := crypto.Keccak256(e.rlpWithoutNonce())
+	rlp := e.rlpWithoutNonce()
+	h := crypto.Keccak256(rlp)
 	copy(buf[:32], h)
 	binary.BigEndian.PutUint64(buf[56:], e.Nonce)
-	d := new(big.Int).SetBytes(crypto.Keccak256(buf))
-	firstBit := math.FirstBitSet(d)
-	x := gmath.Pow(2, float64(firstBit))
-	x /= float64(e.size())
+	powHash := new(big.Int).SetBytes(crypto.Keccak256(buf))
+	leadingZeroes := 256 - powHash.BitLen()
+	x := gmath.Pow(2, float64(leadingZeroes))
+	x /= float64(len(rlp))
 	x /= float64(e.TTL + diff)
 	e.pow = x
 }
diff --git a/whisper/whisperv6/envelope_test.go b/whisper/whisperv6/envelope_test.go
index 410b250a3..d6e38e683 100644
--- a/whisper/whisperv6/envelope_test.go
+++ b/whisper/whisperv6/envelope_test.go
@@ -25,6 +25,33 @@ import (
 	"github.com/ethereum/go-ethereum/crypto"
 )
 
+func TestPoWCalculationsWithNoLeadingZeros(t *testing.T) {
+	e := Envelope{
+		TTL:   1,
+		Data:  []byte{0xde, 0xad, 0xbe, 0xef},
+		Nonce: 100000,
+	}
+
+	e.calculatePoW(0)
+
+	if e.pow != 0.07692307692307693 {
+		t.Fatalf("invalid PoW calculation. Expected 0.07692307692307693, got %v", e.pow)
+	}
+}
+
+func TestPoWCalculationsWith8LeadingZeros(t *testing.T) {
+	e := Envelope{
+		TTL:   1,
+		Data:  []byte{0xde, 0xad, 0xbe, 0xef},
+		Nonce: 48159,
+	}
+	e.calculatePoW(0)
+
+	if e.pow != 40329.846153846156 {
+		t.Fatalf("invalid PoW calculation. Expected 0.07692307692307693, got %v", e.pow)
+	}
+}
+
 func TestEnvelopeOpenAcceptsOnlyOneKeyTypeInFilter(t *testing.T) {
 	symKey := make([]byte, aesKeyLength)
 	mrand.Read(symKey)