diff --git a/README.hbs b/README.hbs
index ec3a1ff..e009f5b 100644
--- a/README.hbs
+++ b/README.hbs
@@ -44,13 +44,24 @@ By default, `byteSize` converts the input number to a human readable string with
 { value: '1.6', unit: 'kB', long: 'kilobytes' }
 ```
 
-The object returned by `byteSize` defines a `toString` method therefore can be used directly in string context (you can override the default behaviour by setting [`options.toStringFn`](https://github.com/75lb/byte-size#bytesizebytes-options--object-)).
+The object returned by `byteSize` defines a `toString` method therefore can be used directly in string context.
 
 ```js
 > `Filesize: ${byteSize(12400)}`
 'Filesize: 12.4 kB'
 ```
 
+Override the default `toString` behaviour by setting [`options.toStringFn`](https://github.com/75lb/byte-size#bytesizebytes-options--object-).
+
+```js
+> function toStringFn () {
+    return `**${this.value}${this.unit}**`
+  }
+
+> `Filesize: ${byteSize(12400, { toStringFn })}`
+'Filesize: **12.4kB**'
+```
+
 Beside the default of `metric`, there are three other built-in units available: `metric_octet`, `iec` and `iec_octet`.
 
 ```js
@@ -92,6 +103,29 @@ Define custom units by passing an object containing one or more additional conve
 '10.0K'
 ```
 
+Override the built-in defaults for the duration of the process by passing an options object to `byteSize.defaultOptions`. This results in cleaner code in cases where `byteSize` is used often with the same options.
+
+```js
+> byteSize.defaultOptions({
+  units: 'simple',
+  precision: 2,
+  customUnits: {
+    simple: [
+      { from: 0, to: 1e3, unit: '' },
+      { from: 1e3, to: 1e6, unit: 'k' },
+      { from: 1e6, to: 1e9, unit: 'm' },
+      { from: 1e9, to: 1e12, unit: 'bn' },
+    ]
+  },
+  toStringFn: function () {
+    return this.value + this.unit
+  }
+})
+
+> [2400, 16400, 3991200].map(byteSize).join(', ')
+2.40k, 16.40k, 3.99m
+```
+
 {{>main}}
 
 ## Load anywhere
diff --git a/README.md b/README.md
index c867237..5603955 100644
--- a/README.md
+++ b/README.md
@@ -44,13 +44,24 @@ By default, `byteSize` converts the input number to a human readable string with
 { value: '1.6', unit: 'kB', long: 'kilobytes' }
 ```
 
-The object returned by `byteSize` defines a `toString` method therefore can be used directly in string context (you can override the default behaviour by setting [`options.toStringFn`](https://github.com/75lb/byte-size#bytesizebytes-options--object-)).
+The object returned by `byteSize` defines a `toString` method therefore can be used directly in string context.
 
 ```js
 > `Filesize: ${byteSize(12400)}`
 'Filesize: 12.4 kB'
 ```
 
+Override the default `toString` behaviour by setting [`options.toStringFn`](https://github.com/75lb/byte-size#bytesizebytes-options--object-).
+
+```js
+> function toStringFn () {
+    return `**${this.value}${this.unit}**`
+  }
+
+> `Filesize: ${byteSize(12400, { toStringFn })}`
+'Filesize: **12.4kB**'
+```
+
 Beside the default of `metric`, there are three other built-in units available: `metric_octet`, `iec` and `iec_octet`.
 
 ```js
@@ -92,6 +103,29 @@ Define custom units by passing an object containing one or more additional conve
 '10.0K'
 ```
 
+Override the built-in defaults for the duration of the process by passing an options object to `byteSize.defaultOptions`. This results in cleaner code in cases where `byteSize` is used often with the same options.
+
+```js
+> byteSize.defaultOptions({
+  units: 'simple',
+  precision: 2,
+  customUnits: {
+    simple: [
+      { from: 0, to: 1e3, unit: '' },
+      { from: 1e3, to: 1e6, unit: 'k' },
+      { from: 1e6, to: 1e9, unit: 'm' },
+      { from: 1e9, to: 1e12, unit: 'bn' },
+    ]
+  },
+  toStringFn: function () {
+    return this.value + this.unit
+  }
+})
+
+> [2400, 16400, 3991200].map(byteSize).join(', ')
+2.40k, 16.40k, 3.99m
+```
+
 <a name="module_byte-size"></a>
 
 ## byte-size
@@ -119,11 +153,13 @@ Returns an object with the spec `{ value: string, unit: string, long: string }`.
 <a name="module_byte-size--byteSize.defaultOptions"></a>
 
 #### byteSize.defaultOptions([options])
+Set the default `byteSize` options for the duration of the process.
+
 **Kind**: static method of [<code>byteSize</code>](#exp_module_byte-size--byteSize)  
 
 | Param | Type | Description |
 | --- | --- | --- |
-| [options] | <code>object</code> | Default options. |
+| [options] | <code>object</code> | A `byteSize` options object. |
 
 
 ## Load anywhere
diff --git a/dist/index.js b/dist/index.js
index 30cadcc..eec1d45 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -9,6 +9,7 @@
    */
 
   let defaultOptions = {};
+  const _options = new WeakMap();
 
   class ByteSize {
     constructor (bytes, options) {
@@ -16,7 +17,7 @@
         units: 'metric',
         precision: 1
       }, defaultOptions, options);
-      this.options = options;
+      _options.set(this, options);
 
       const tables = {
         metric: [
@@ -89,7 +90,8 @@
     }
 
     toString () {
-      return this.options.toStringFn ? this.options.toStringFn.bind(this)() : `${this.value} ${this.unit}`
+      const options = _options.get(this);
+      return options.toStringFn ? options.toStringFn.bind(this)() : `${this.value} ${this.unit}`
     }
   }
 
@@ -109,7 +111,8 @@
   }
 
   /**
-   * @param [options] {object} - Default options.
+   * Set the default `byteSize` options for the duration of the process.
+   * @param [options] {object} - A `byteSize` options object.
    */
   byteSize.defaultOptions = function (options) {
     defaultOptions = options;
diff --git a/index.mjs b/index.mjs
index 2aefd31..cf8bd02 100644
--- a/index.mjs
+++ b/index.mjs
@@ -3,6 +3,7 @@
  */
 
 let defaultOptions = {}
+const _options = new WeakMap()
 
 class ByteSize {
   constructor (bytes, options) {
@@ -10,7 +11,7 @@ class ByteSize {
       units: 'metric',
       precision: 1
     }, defaultOptions, options)
-    this.options = options
+    _options.set(this, options)
 
     const tables = {
       metric: [
@@ -83,7 +84,8 @@ class ByteSize {
   }
 
   toString () {
-    return this.options.toStringFn ? this.options.toStringFn.bind(this)() : `${this.value} ${this.unit}`
+    const options = _options.get(this)
+    return options.toStringFn ? options.toStringFn.bind(this)() : `${this.value} ${this.unit}`
   }
 }
 
@@ -103,7 +105,8 @@ function byteSize (bytes, options) {
 }
 
 /**
- * @param [options] {object} - Default options.
+ * Set the default `byteSize` options for the duration of the process.
+ * @param [options] {object} - A `byteSize` options object.
  */
 byteSize.defaultOptions = function (options) {
   defaultOptions = options