diff --git a/lib/bcoin/mempool.js b/lib/bcoin/mempool.js
index f550c91f..8aa25dce 100644
--- a/lib/bcoin/mempool.js
+++ b/lib/bcoin/mempool.js
@@ -1472,6 +1472,11 @@ Mempool.prototype._removeSpenders = function _removeSpenders(entry) {
 
 /**
  * Calculate the memory usage of a transaction.
+ * Note that this only calculates the JS heap
+ * size. Sizes of buffers are ignored (the v8
+ * heap is what we care most about). All numbers
+ * are based on the output of v8 heap snapshots
+ * of TX objects.
  * @param {TX} tx
  * @returns {Number} Usage in bytes.
  */
@@ -1480,38 +1485,60 @@ Mempool.prototype.memUsage = function memUsage(tx) {
   var mem = 0;
   var i, j, input, output;
 
-  mem += 8;
-  mem += 8;
-  mem += 8;
+  mem += 264; // tx
+  mem += 80; // _hash
+  mem += 80; // _raw
+
+  mem += 32; // input array
 
   for (i = 0; i < tx.inputs.length; i++) {
     input = tx.inputs[i];
+
+    mem += 144; // input
+
     if (input.coin) {
-      mem += 93;
-      mem += input.coin.script.getSize() * 2;
+      mem += 144; // coin
+      mem += 88; // coin hash
+      mem += 40; // script
+      mem += 80; // script raw buffer
+      mem += 32; // script code array
+      mem += input.coin.script.code.length * 40; // opcodes
+      mem += 80; // add a code buffer (assume p2pkh)
     }
-    mem += 76;
-    mem += input.script.getSize() * 2;
-    for (j = 0; j < input.witness.items.length; j++)
-      mem += input.witness.items[j].length;
+
+    mem += 104; // prevout
+    mem += 88; // prevout hash
+
+    mem += 40; // script
+    mem += 80; // script raw buffer
+    mem += 32; // script code array
+    mem += input.script.code.length * 40; // opcodes
+    mem += 160; // add two code buffers (assume p2pkh)
+
+    mem += 96; // witness
+    mem += 32; // witness items
+    mem += input.witness.items.length * 80; // witness buffers
   }
 
+  mem += 32; // output array
+
   for (i = 0; i < tx.outputs.length; i++) {
     output = tx.outputs[i];
-    mem += 8;
-    mem += output.script.getSize() * 2;
+    mem += 120; // output
+    mem += 40; // script
+    mem += 80; // script raw buffer
+    mem += 32; // script code array
+    mem += output.script.code.length * 40; // opcodes
+    mem += 80; // add a code buffer (assume p2pkh)
   }
 
-  mem += 8;
+  mem += 176; // mempool entry
 
-  // Mempool Entry
-  mem += 40;
-
-  // Map entry
-  mem += 64;
-
-  // Spent entry
-  mem += 64;
+  // Note: normally we could add 176 (88 * 2)
+  // bytes for the spent and tx hash table
+  // entries, but v8 seems to store these
+  // strings off the heap, so it shouldn't
+  // be too much of a problem.
 
   return mem;
 };