diff --git a/src/index.js b/src/index.js
index 47448a08..383e63fd 100644
--- a/src/index.js
+++ b/src/index.js
@@ -252,13 +252,6 @@ async function getEntries({ input, cwd }) {
 	return entries;
 }
 
-function replaceName(filename, name) {
-	return resolve(
-		dirname(filename),
-		name + basename(filename).replace(/^[^.]+/, ''),
-	);
-}
-
 function walk(exports) {
 	if (typeof exports === 'string') return exports;
 	return walk(exports['.'] || exports.import || exports.module);
@@ -288,26 +281,23 @@ function getMain({ options, entry, format }) {
 
 	const mainsByFormat = {};
 
-	mainsByFormat.es = replaceName(
+	mainsByFormat.es = resolve(
 		pkg.module && !pkg.module.match(/src\//)
 			? pkg.module
-			: pkg['jsnext:main'] || 'x.esm.js',
-		mainNoExtension,
+			: pkg['jsnext:main'] || `${mainNoExtension}.esm.js`,
 	);
-	mainsByFormat.modern = replaceName(
+	mainsByFormat.modern = resolve(
 		(pkg.exports && walk(pkg.exports)) ||
 			(pkg.syntax && pkg.syntax.esmodules) ||
 			pkg.esmodule ||
-			'x.modern.js',
-		mainNoExtension,
+			`${mainNoExtension}.modern.js`,
 	);
-	mainsByFormat.cjs = replaceName(
-		pkg['cjs:main'] || (pkg.type && pkg.type === 'module' ? 'x.cjs' : 'x.js'),
-		mainNoExtension,
+	mainsByFormat.cjs = resolve(
+		pkg['cjs:main'] ||
+			`${mainNoExtension}.${pkg.type === 'module' ? 'c' : ''}js`,
 	);
-	mainsByFormat.umd = replaceName(
-		pkg['umd:main'] || pkg.unpkg || 'x.umd.js',
-		mainNoExtension,
+	mainsByFormat.umd = resolve(
+		pkg['umd:main'] || pkg.unpkg || `${mainNoExtension}.umd.js`,
 	);
 
 	return mainsByFormat[format] || mainsByFormat.cjs;
diff --git a/test/__snapshots__/index.test.js.snap b/test/__snapshots__/index.test.js.snap
index e70c9176..88981a40 100644
--- a/test/__snapshots__/index.test.js.snap
+++ b/test/__snapshots__/index.test.js.snap
@@ -1417,6 +1417,122 @@ exports[`fixtures build custom-babelrc with microbundle 5`] = `
 "
 `;
 
+exports[`fixtures build custom-output with microbundle 1`] = `
+"Used script: microbundle -f cjs,es,umd,modern
+
+Directory tree:
+
+custom-output
+  dist
+    custom-name.js
+    custom-name.js.map
+    index.esm.js
+    index.esm.js.map
+    index.js
+    index.js.map
+    index.umd.js
+    index.umd.js.map
+  package.json
+  src
+    index.js
+    two.js
+
+
+Build \\"customOutput\\" to dist:
+187 B: index.js.gz
+138 B: index.js.br
+188 B: index.esm.js.gz
+139 B: index.esm.js.br
+274 B: index.umd.js.gz
+203 B: index.umd.js.br
+113 B: custom-name.js.gz
+92 B: custom-name.js.br"
+`;
+
+exports[`fixtures build custom-output with microbundle 2`] = `8`;
+
+exports[`fixtures build custom-output with microbundle 3`] = `
+"async function n(...n){return n.reduce((n,t)=>n+t,0)}async function t(...t){return[await n(...t),await n(...t)]}export default t;
+//# sourceMappingURL=custom-name.js.map
+"
+`;
+
+exports[`fixtures build custom-output with microbundle 4`] = `
+"var r=function(){try{var r=arguments;return Promise.resolve([].slice.call(r).reduce(function(r,e){return r+e},0))}catch(r){return Promise.reject(r)}};export default function(){try{var e=arguments,t=[].slice.call(e);return Promise.resolve(r.apply(void 0,t)).then(function(e){return Promise.resolve(r.apply(void 0,t)).then(function(r){return[e,r]})})}catch(r){return Promise.reject(r)}}
+//# sourceMappingURL=index.esm.js.map
+"
+`;
+
+exports[`fixtures build custom-output with microbundle 5`] = `
+"var r=function(){try{var r=arguments;return Promise.resolve([].slice.call(r).reduce(function(r,e){return r+e},0))}catch(r){return Promise.reject(r)}};module.exports=function(){try{var e=arguments,t=[].slice.call(e);return Promise.resolve(r.apply(void 0,t)).then(function(e){return Promise.resolve(r.apply(void 0,t)).then(function(r){return[e,r]})})}catch(r){return Promise.reject(r)}};
+//# sourceMappingURL=index.js.map
+"
+`;
+
+exports[`fixtures build custom-output with microbundle 6`] = `
+"!function(e,t){\\"object\\"==typeof exports&&\\"undefined\\"!=typeof module?module.exports=t():\\"function\\"==typeof define&&define.amd?define(t):(e||self).customOutput=t()}(this,function(){var e=function(){try{var e=arguments;return Promise.resolve([].slice.call(e).reduce(function(e,t){return e+t},0))}catch(e){return Promise.reject(e)}};return function(){try{var t=arguments,r=[].slice.call(t);return Promise.resolve(e.apply(void 0,r)).then(function(t){return Promise.resolve(e.apply(void 0,r)).then(function(e){return[t,e]})})}catch(e){return Promise.reject(e)}}});
+//# sourceMappingURL=index.umd.js.map
+"
+`;
+
+exports[`fixtures build custom-outputs with microbundle 1`] = `
+"Used script: microbundle -f cjs,es,umd,modern
+
+Directory tree:
+
+custom-outputs
+  dist
+    custom-exports.js
+    custom-exports.js.map
+    custom-main.js
+    custom-main.js.map
+    custom-module.js
+    custom-module.js.map
+    custom-unpkg.js
+    custom-unpkg.js.map
+  package.json
+  src
+    index.js
+    two.js
+
+
+Build \\"customOutputs\\" to dist:
+187 B: custom-main.js.gz
+138 B: custom-main.js.br
+188 B: custom-module.js.gz
+139 B: custom-module.js.br
+273 B: custom-unpkg.js.gz
+203 B: custom-unpkg.js.br
+113 B: custom-exports.js.gz
+92 B: custom-exports.js.br"
+`;
+
+exports[`fixtures build custom-outputs with microbundle 2`] = `8`;
+
+exports[`fixtures build custom-outputs with microbundle 3`] = `
+"async function n(...n){return n.reduce((n,t)=>n+t,0)}async function t(...t){return[await n(...t),await n(...t)]}export default t;
+//# sourceMappingURL=custom-exports.js.map
+"
+`;
+
+exports[`fixtures build custom-outputs with microbundle 4`] = `
+"var r=function(){try{var r=arguments;return Promise.resolve([].slice.call(r).reduce(function(r,e){return r+e},0))}catch(r){return Promise.reject(r)}};module.exports=function(){try{var e=arguments,t=[].slice.call(e);return Promise.resolve(r.apply(void 0,t)).then(function(e){return Promise.resolve(r.apply(void 0,t)).then(function(r){return[e,r]})})}catch(r){return Promise.reject(r)}};
+//# sourceMappingURL=custom-main.js.map
+"
+`;
+
+exports[`fixtures build custom-outputs with microbundle 5`] = `
+"var r=function(){try{var r=arguments;return Promise.resolve([].slice.call(r).reduce(function(r,e){return r+e},0))}catch(r){return Promise.reject(r)}};export default function(){try{var e=arguments,t=[].slice.call(e);return Promise.resolve(r.apply(void 0,t)).then(function(e){return Promise.resolve(r.apply(void 0,t)).then(function(r){return[e,r]})})}catch(r){return Promise.reject(r)}}
+//# sourceMappingURL=custom-module.js.map
+"
+`;
+
+exports[`fixtures build custom-outputs with microbundle 6`] = `
+"!function(e,t){\\"object\\"==typeof exports&&\\"undefined\\"!=typeof module?module.exports=t():\\"function\\"==typeof define&&define.amd?define(t):(e||self).customOutputs=t()}(this,function(){var e=function(){try{var e=arguments;return Promise.resolve([].slice.call(e).reduce(function(e,t){return e+t},0))}catch(e){return Promise.reject(e)}};return function(){try{var t=arguments,r=[].slice.call(t);return Promise.resolve(e.apply(void 0,r)).then(function(t){return Promise.resolve(e.apply(void 0,r)).then(function(e){return[t,e]})})}catch(e){return Promise.reject(e)}}});
+//# sourceMappingURL=custom-unpkg.js.map
+"
+`;
+
 exports[`fixtures build custom-source with microbundle 1`] = `
 "Used script: microbundle
 
diff --git a/test/fixtures/custom-output/package.json b/test/fixtures/custom-output/package.json
new file mode 100644
index 00000000..a80ba852
--- /dev/null
+++ b/test/fixtures/custom-output/package.json
@@ -0,0 +1,8 @@
+{
+  "name": "custom-output",
+  "main": "./dist/index.js",
+  "exports": "./dist/custom-name.js",
+  "scripts": {
+    "build": "microbundle -f cjs,es,umd,modern"
+  }
+}
diff --git a/test/fixtures/custom-output/src/index.js b/test/fixtures/custom-output/src/index.js
new file mode 100644
index 00000000..c0a2efb0
--- /dev/null
+++ b/test/fixtures/custom-output/src/index.js
@@ -0,0 +1,5 @@
+import { two } from './two';
+
+export default async function (...args) {
+	return [await two(...args), await two(...args)];
+}
diff --git a/test/fixtures/custom-output/src/two.js b/test/fixtures/custom-output/src/two.js
new file mode 100644
index 00000000..9f1f741b
--- /dev/null
+++ b/test/fixtures/custom-output/src/two.js
@@ -0,0 +1,3 @@
+export async function two(...args) {
+	return args.reduce((total, value) => total + value, 0);
+}
diff --git a/test/fixtures/custom-outputs/package.json b/test/fixtures/custom-outputs/package.json
new file mode 100644
index 00000000..b1102082
--- /dev/null
+++ b/test/fixtures/custom-outputs/package.json
@@ -0,0 +1,10 @@
+{
+  "name": "custom-outputs",
+  "main": "./dist/custom-main.js",
+  "module": "./dist/custom-module.js",
+  "unpkg": "./dist/custom-unpkg.js",
+  "exports": "./dist/custom-exports.js",
+  "scripts": {
+    "build": "microbundle -f cjs,es,umd,modern"
+  }
+}
diff --git a/test/fixtures/custom-outputs/src/index.js b/test/fixtures/custom-outputs/src/index.js
new file mode 100644
index 00000000..c0a2efb0
--- /dev/null
+++ b/test/fixtures/custom-outputs/src/index.js
@@ -0,0 +1,5 @@
+import { two } from './two';
+
+export default async function (...args) {
+	return [await two(...args), await two(...args)];
+}
diff --git a/test/fixtures/custom-outputs/src/two.js b/test/fixtures/custom-outputs/src/two.js
new file mode 100644
index 00000000..9f1f741b
--- /dev/null
+++ b/test/fixtures/custom-outputs/src/two.js
@@ -0,0 +1,3 @@
+export async function two(...args) {
+	return args.reduce((total, value) => total + value, 0);
+}