diff --git a/nip57.test.ts b/nip57.test.ts
index a5608ac..93c79c6 100644
--- a/nip57.test.ts
+++ b/nip57.test.ts
@@ -1,7 +1,14 @@
 import { describe, test, expect, mock } from 'bun:test'
 import { finalizeEvent } from './pure.ts'
 import { getPublicKey, generateSecretKey } from './pure.ts'
-import { getZapEndpoint, makeZapReceipt, makeZapRequest, useFetchImplementation, validateZapRequest } from './nip57.ts'
+import {
+  getSatoshisAmountFromBolt11,
+  getZapEndpoint,
+  makeZapReceipt,
+  makeZapRequest,
+  useFetchImplementation,
+  validateZapRequest,
+} from './nip57.ts'
 import { buildEvent } from './test-helpers.ts'
 
 describe('getZapEndpoint', () => {
@@ -317,3 +324,26 @@ describe('makeZapReceipt', () => {
     expect(JSON.stringify(result.tags)).not.toContain('preimage')
   })
 })
+
+test('parses the amount from bolt11 invoices', () => {
+  expect(
+    getSatoshisAmountFromBolt11(
+      'lnbc4u1p5zcarnpp5djng98r73nxu66nxp6gndjkw24q7rdzgp7p80lt0gk4z3h3krkssdq9tfpygcqzzsxqzjcsp58hz3v5qefdm70g5fnm2cn6q9thzpu6m4f5wjqurhur5xzmf9vl3s9qxpqysgq9v6qv86xaruzeak9jjyz54fygrkn526z7xhm0llh8wl44gcgh0rznhjqdswd4cjurzdgh0pgzrfj4sd7f3mf89jd6kadse008ex7kxgqqa5xrk',
+    ),
+  ).toEqual(400)
+  expect(
+    getSatoshisAmountFromBolt11(
+      'lnbc8400u1p5zcaz5pp5ltvyhtg4ed7sd8jurj28ugmavezkmqsadpe3t9npufpcrd0uet0scqzyssp5l3hz4ayt5ee0p83ma4a96l2rruhx33eyycewldu2ffa5pk2qx7jq9q7sqqqqqqqqqqqqqqqqqqqsqqqqqysgqdq8w3jhxaqmqz9gxqyjw5qrzjqwryaup9lh50kkranzgcdnn2fgvx390wgj5jd07rwr3vxeje0glclll8qkt3np4rqyqqqqlgqqqqqeqqjqhuhjk5u9r850ncxngne7cfp9s08s2nm6c2rkz7jhl8gjmlx0fga5tlncgeuh4avlsrkq6ljyyhgq8rrxprga03esqhd0gf5455x6tdcqahhw9q',
+    ),
+  ).toEqual(840000)
+  expect(
+    getSatoshisAmountFromBolt11(
+      'lnbc210n1p5zcuaxpp52nn778cfk46md4ld0hdj2juuzvfrsrdaf4ek2k0yeensae07x2cqdq9tfpygcqzzsxqzjcsp5768c4k79jtnq92pgppan8rjnujcpcqhnqwqwk3lm5dfr7e0k2a7s9qxpqysgqt8lnh9l7ple27t73x7gty570ltas2s33uahc7egke5tdmhxr3ezn590wf2utxyt7d3afnk2lxc2u0enc6n53ck4mxwpmzpxa7ws05aqp0c5x3r',
+    ),
+  ).toEqual(21)
+  expect(
+    getSatoshisAmountFromBolt11(
+      'lnbc899640n1p5zcuavpp5w72fqrf09286lq33vw364qryrq5nw60z4dhdx56f8w05xkx4massdq9tfpygcqzzsxqzjcsp5qrqn4kpvem5jwpl63kj5pfdlqxg2plaffz0prz7vaqjy29uc66us9qxpqysgqlhzzqmn2jxd2476404krm8nvrarymwq7nj2zecl92xug54ek0mfntdxvxwslf756m8kq0r7jtpantm52fmewc72r5lfmd85505jnemgqw5j0pc',
+    ),
+  ).toEqual(89964)
+})
diff --git a/nip57.ts b/nip57.ts
index 0736e2f..6ff1b8a 100644
--- a/nip57.ts
+++ b/nip57.ts
@@ -77,7 +77,7 @@ export function makeZapRequest({
     if (isReplaceableKind(event.kind)) {
       const a = ['a', `${event.kind}:${event.pubkey}:`]
       zr.tags.push(a)
-    // addressable event
+      // addressable event
     } else if (isAddressableKind(event.kind)) {
       let d = event.tags.find(([t, v]) => t === 'd' && v)
       if (!d) throw new Error('d tag not found or is empty')
@@ -142,3 +142,52 @@ export function makeZapReceipt({
 
   return zap
 }
+
+export function getSatoshisAmountFromBolt11(bolt11: string): number {
+  if (bolt11.length < 50) {
+    return 0
+  }
+  bolt11 = bolt11.substring(0, 50)
+  const idx = bolt11.lastIndexOf('1')
+  if (idx === -1) {
+    return 0
+  }
+  const hrp = bolt11.substring(0, idx)
+  if (!hrp.startsWith('lnbc')) {
+    return 0
+  }
+  const amount = hrp.substring(4) // equivalent to strings.CutPrefix
+
+  if (amount.length < 1) {
+    return 0
+  }
+
+  // if last character is a digit, then the amount can just be interpreted as BTC
+  const char = amount[amount.length - 1]
+  const digit = char.charCodeAt(0) - '0'.charCodeAt(0)
+  const isDigit = digit >= 0 && digit <= 9
+
+  let cutPoint = amount.length - 1
+  if (isDigit) {
+    cutPoint++
+  }
+
+  if (cutPoint < 1) {
+    return 0
+  }
+
+  const num = parseInt(amount.substring(0, cutPoint))
+
+  switch (char) {
+    case 'm':
+      return num * 100000
+    case 'u':
+      return num * 100
+    case 'n':
+      return num / 10
+    case 'p':
+      return num / 10000
+    default:
+      return num * 100000000
+  }
+}