diff --git a/src/locale.js b/src/locale.js
index 404f941..7dcf7d7 100644
--- a/src/locale.js
+++ b/src/locale.js
@@ -20,7 +20,7 @@ export default function(locale) {
       minus = locale.minus === undefined ? "−" : locale.minus + "",
       nan = locale.nan === undefined ? "NaN" : locale.nan + "";
 
-  function newFormat(specifier) {
+  function newFormat(specifier, options) {
     specifier = formatSpecifier(specifier);
 
     var fill = specifier.fill,
@@ -45,8 +45,8 @@ export default function(locale) {
 
     // Compute the prefix and suffix.
     // For SI-prefix, the suffix is lazily computed.
-    var prefix = symbol === "$" ? currencyPrefix : symbol === "#" && /[boxX]/.test(type) ? "0" + type.toLowerCase() : "",
-        suffix = symbol === "$" ? currencySuffix : /[%p]/.test(type) ? percent : "";
+    var prefix = (options?.prefix !== undefined ? options.prefix : "") + (symbol === "$" ? currencyPrefix : symbol === "#" && /[boxX]/.test(type) ? "0" + type.toLowerCase() : ""),
+        suffix = (symbol === "$" ? currencySuffix : /[%p]/.test(type) ? percent : "") + (options?.suffix !== undefined ? options.suffix : "");
 
     // What format function should we use?
     // Is this an integer type?
@@ -132,12 +132,11 @@ export default function(locale) {
   }
 
   function formatPrefix(specifier, value) {
-    var f = newFormat((specifier = formatSpecifier(specifier), specifier.type = "f", specifier)),
-        e = Math.max(-8, Math.min(8, Math.floor(exponent(value) / 3))) * 3,
+    var e = Math.max(-8, Math.min(8, Math.floor(exponent(value) / 3))) * 3,
         k = Math.pow(10, -e),
-        prefix = prefixes[8 + e / 3];
+        f = newFormat((specifier = formatSpecifier(specifier), specifier.type = "f", specifier), {suffix: prefixes[8 + e / 3]});
     return function(value) {
-      return f(k * value) + prefix;
+      return f(k * value);
     };
   }
 
diff --git a/test/formatPrefix-test.js b/test/formatPrefix-test.js
index a46766c..69308ef 100644
--- a/test/formatPrefix-test.js
+++ b/test/formatPrefix-test.js
@@ -1,22 +1,39 @@
 import {assert, test} from "vitest";
-import {formatPrefix} from "../src/index.js";
+import {format, formatPrefix} from "../src/index.js";
 
-test("formatPrefix(\"s\", value)(number) formats with the SI prefix appropriate to the specified value", () => {
+test("formatPrefix(\",.0s\", value)(number) formats with the SI prefix appropriate to the specified value", () => {
   assert.strictEqual(formatPrefix(",.0s", 1e-6)(.00042), "420µ");
   assert.strictEqual(formatPrefix(",.0s", 1e-6)(.0042), "4,200µ");
+});
+
+test("formatPrefix(\",.3s\", value)(number) formats with the SI prefix appropriate to the specified value", () => {
   assert.strictEqual(formatPrefix(",.3s", 1e-3)(.00042), "0.420m");
 });
 
-test("formatPrefix(\"s\", value)(number) uses yocto for very small reference values", () => {
+test("formatPrefix(\",.0s\", value)(number) uses yocto for very small reference values", () => {
   assert.strictEqual(formatPrefix(",.0s", 1e-27)(1e-24), "1y");
 });
 
-test("formatPrefix(\"s\", value)(number) uses yotta for very small reference values", () => {
+test("formatPrefix(\",.0s\", value)(number) uses yotta for very small reference values", () => {
   assert.strictEqual(formatPrefix(",.0s", 1e27)(1e24), "1Y");
 });
 
-test("formatPrefix(\"$,s\", value)(number) formats with the specified SI prefix", () => {
+test("formatPrefix(\" $12,.1s\", value)(number) formats with the specified SI prefix", () => {
+  // The fixed length of 12 is inclusive of the unit 'M'
   const f = formatPrefix(" $12,.1s", 1e6);
-  assert.strictEqual(f(-42e6),  "      −$42.0M");
-  assert.strictEqual(f(+4.2e6), "        $4.2M");
+  assert.strictEqual(f(-42e6),  "     −$42.0M");
+  assert.strictEqual(f(+4.2e6), "       $4.2M");
+});
+
+test("formatPrefix(\" $12,.1s\", value)(number) matches format(\" $12,.2s\")(number) when the units are the same", () => {
+  // The fixed length of 12 is inclusive of the unit 'M'
+  const fp = formatPrefix(" $12,.1s", 1e6);
+  const f = format(" $12,.2s");
+  assert.strictEqual(fp(+4.2e6), "       $4.2M");
+  assert.strictEqual(f(+4.2e6),  "       $4.2M");
+});
+
+test("formatPrefix(\"($~s\", value)(number) formats with the SI prefix inside parentheses", () => {
+  assert.strictEqual(formatPrefix("($~s", 1e3)(1e3), "$1k");
+  assert.strictEqual(formatPrefix("($~s", 1e3)(-1e3), "($1k)");
 });