diff --git a/docs/api.md b/docs/api.md
index d5d36ad8..519953bc 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -374,15 +374,15 @@ When used without parameters, performs a fast, mild blur of the output image. Th
 
 When a `sigma` is provided, performs a slower, more accurate Gaussian blur. This typically reduces performance by 25%.
 
-* `sigma`, if present, is a Number between 0.3 and 1000 representing the approximate blur radius in pixels.
+* `sigma`, if present, is a Number between 0.3 and 1000 representing the sigma of the Gaussian mask, where `sigma = 1 + radius / 2`.
 
-#### sharpen([radius], [flat], [jagged])
+#### sharpen([sigma], [flat], [jagged])
 
 When used without parameters, performs a fast, mild sharpen of the output image. This typically reduces performance by 10%.
 
-When a `radius` is provided, performs a slower, more accurate sharpen of the L channel in the LAB colour space. Separate control over the level of sharpening in "flat" and "jagged" areas is available. This typically reduces performance by 50%.
+When a `sigma` is provided, performs a slower, more accurate sharpen of the L channel in the LAB colour space. Separate control over the level of sharpening in "flat" and "jagged" areas is available. This typically reduces performance by 50%.
 
-* `radius`, if present, is an integral Number representing the sharpen mask radius in pixels.
+* `sigma`, if present, is a Number representing the sigma of the Gaussian mask, where `sigma = 1 + radius / 2`.
 * `flat`, if present, is a Number representing the level of sharpening to apply to "flat" areas, defaulting to a value of 1.0.
 * `jagged`, if present, is a Number representing the level of sharpening to apply to "jagged" areas, defaulting to a value of 2.0.
 
diff --git a/docs/changelog.md b/docs/changelog.md
index 72e4dc03..d53e6be9 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -4,6 +4,7 @@
 
 * Take advantage of libvips 8.3 features.
   Use shrink-on-load for WebP input.
+  Break existing sharpen API to accept sigma and improve precision.
   [#369](https://github.com/lovell/sharp/issues/369)
 
 ### v0.14 - "*needle*"
diff --git a/index.js b/index.js
index 94e8a7e7..36d0fabe 100644
--- a/index.js
+++ b/index.js
@@ -80,7 +80,7 @@ var Sharp = function(input, options) {
     flatten: false,
     negate: false,
     blurSigma: 0,
-    sharpenRadius: 0,
+    sharpenSigma: 0,
     sharpenFlat: 1,
     sharpenJagged: 2,
     threshold: 0,
@@ -154,14 +154,20 @@ var isDefined = function(val) {
 var isObject = function(val) {
   return typeof val === 'object';
 };
+var isBoolean = function(val) {
+  return typeof val === 'boolean';
+};
 var isBuffer = function(val) {
   return typeof val === 'object' && val instanceof Buffer;
 };
 var isString = function(val) {
   return typeof val === 'string' && val.length > 0;
 };
+var isNumber = function(val) {
+  return typeof val === 'number' && !Number.isNaN(val);
+};
 var isInteger = function(val) {
-  return typeof val === 'number' && !Number.isNaN(val) && val % 1 === 0;
+  return isNumber(val) && val % 1 === 0;
 };
 var inRange = function(val, min, max) {
   return val >= min && val <= max;
@@ -406,17 +412,17 @@ Sharp.prototype.withoutEnlargement = function(withoutEnlargement) {
   Call with a sigma to use a slower, more accurate Gaussian blur.
 */
 Sharp.prototype.blur = function(sigma) {
-  if (typeof sigma === 'undefined') {
+  if (!isDefined(sigma)) {
     // No arguments: default to mild blur
     this.options.blurSigma = -1;
-  } else if (typeof sigma === 'boolean') {
+  } else if (isBoolean(sigma)) {
     // Boolean argument: apply mild blur?
     this.options.blurSigma = sigma ? -1 : 0;
-  } else if (typeof sigma === 'number' && !Number.isNaN(sigma) && sigma >= 0.3 && sigma <= 1000) {
+  } else if (isNumber(sigma) && inRange(sigma, 0.3, 1000)) {
     // Numeric argument: specific sigma
     this.options.blurSigma = sigma;
   } else {
-    throw new Error('Invalid blur sigma (0.3 to 1000.0) ' + sigma);
+    throw new Error('Invalid blur sigma (0.3 - 1000.0) ' + sigma);
   }
   return this;
 };
@@ -425,38 +431,38 @@ Sharp.prototype.blur = function(sigma) {
   Sharpen the output image.
   Call without a radius to use a fast, mild sharpen.
   Call with a radius to use a slow, accurate sharpen using the L of LAB colour space.
-    radius - size of mask in pixels, must be integer
+    sigma - sigma of mask
     flat - level of "flat" area sharpen, default 1
     jagged - level of "jagged" area sharpen, default 2
 */
-Sharp.prototype.sharpen = function(radius, flat, jagged) {
-  if (typeof radius === 'undefined') {
+Sharp.prototype.sharpen = function(sigma, flat, jagged) {
+  if (!isDefined(sigma)) {
     // No arguments: default to mild sharpen
-    this.options.sharpenRadius = -1;
-  } else if (typeof radius === 'boolean') {
+    this.options.sharpenSigma = -1;
+  } else if (isBoolean(sigma)) {
     // Boolean argument: apply mild sharpen?
-    this.options.sharpenRadius = radius ? -1 : 0;
-  } else if (typeof radius === 'number' && !Number.isNaN(radius) && (radius % 1 === 0) && radius >= 1) {
-    // Numeric argument: specific radius
-    this.options.sharpenRadius = radius;
+    this.options.sharpenSigma = sigma ? -1 : 0;
+  } else if (isNumber(sigma) && inRange(sigma, 0.01, 10000)) {
+    // Numeric argument: specific sigma
+    this.options.sharpenSigma = sigma;
     // Control over flat areas
-    if (typeof flat !== 'undefined' && flat !== null) {
-      if (typeof flat === 'number' && !Number.isNaN(flat) && flat >= 0) {
+    if (isDefined(flat)) {
+      if (isNumber(flat) && inRange(flat, 0, 10000)) {
         this.options.sharpenFlat = flat;
       } else {
-        throw new Error('Invalid sharpen level for flat areas ' + flat + ' (expected >= 0)');
+        throw new Error('Invalid sharpen level for flat areas (0 - 10000) ' + flat);
       }
     }
     // Control over jagged areas
-    if (typeof jagged !== 'undefined' && jagged !== null) {
-      if (typeof jagged === 'number' && !Number.isNaN(jagged) && jagged >= 0) {
+    if (isDefined(jagged)) {
+      if (isNumber(jagged) && inRange(jagged, 0, 10000)) {
         this.options.sharpenJagged = jagged;
       } else {
-        throw new Error('Invalid sharpen level for jagged areas ' + jagged + ' (expected >= 0)');
+        throw new Error('Invalid sharpen level for jagged areas (0 - 10000) ' + jagged);
       }
     }
   } else {
-    throw new Error('Invalid sharpen radius ' + radius + ' (expected integer >= 1)');
+    throw new Error('Invalid sharpen sigma (0.01 - 10000) ' + sigma);
   }
   return this;
 };
diff --git a/src/operations.cc b/src/operations.cc
index 555f183d..ca5d41ba 100644
--- a/src/operations.cc
+++ b/src/operations.cc
@@ -135,10 +135,10 @@ namespace sharp {
   }
 
   /*
-   * Gaussian blur (use sigma <0 for fast blur)
+   * Gaussian blur. Use sigma of -1.0 for fast blur.
    */
   VImage Blur(VImage image, double const sigma) {
-    if (sigma < 0.0) {
+    if (sigma == -1.0) {
       // Fast, mild blur - averages neighbouring pixels
       VImage blur = VImage::new_matrixv(3, 3,
         1.0, 1.0, 1.0,
@@ -153,10 +153,10 @@ namespace sharp {
   }
 
   /*
-   * Sharpen flat and jagged areas. Use radius of -1 for fast sharpen.
+   * Sharpen flat and jagged areas. Use sigma of -1.0 for fast sharpen.
    */
-  VImage Sharpen(VImage image, int const radius, double const flat, double const jagged) {
-    if (radius == -1) {
+  VImage Sharpen(VImage image, double const sigma, double const flat, double const jagged) {
+    if (sigma == -1.0) {
       // Fast, mild sharpen
       VImage sharpen = VImage::new_matrixv(3, 3,
         -1.0, -1.0, -1.0,
@@ -167,7 +167,7 @@ namespace sharp {
     } else {
       // Slow, accurate sharpen in LAB colour space, with control over flat vs jagged areas
       return image.sharpen(
-        VImage::option()->set("radius", radius)->set("m1", flat)->set("m2", jagged)
+        VImage::option()->set("sigma", sigma)->set("m1", flat)->set("m2", jagged)
       );
     }
   }
diff --git a/src/operations.h b/src/operations.h
index 48b14834..7820cd2e 100644
--- a/src/operations.h
+++ b/src/operations.h
@@ -25,14 +25,14 @@ namespace sharp {
   VImage Gamma(VImage image, double const exponent);
 
   /*
-   * Gaussian blur. Use sigma of -1 for fast blur.
+   * Gaussian blur. Use sigma of -1.0 for fast blur.
    */
   VImage Blur(VImage image, double const sigma);
 
   /*
-   * Sharpen flat and jagged areas. Use radius of -1 for fast sharpen.
+   * Sharpen flat and jagged areas. Use sigma of -1.0 for fast sharpen.
    */
-  VImage Sharpen(VImage image, int const radius, double const flat, double const jagged);
+  VImage Sharpen(VImage image, double const sigma, double const flat, double const jagged);
 
   /*
     Calculate crop area based on image entropy
diff --git a/src/pipeline.cc b/src/pipeline.cc
index 2fd4fa37..209a2e6d 100644
--- a/src/pipeline.cc
+++ b/src/pipeline.cc
@@ -444,7 +444,7 @@ class PipelineWorker : public AsyncWorker {
 
       bool shouldAffineTransform = xresidual != 1.0 || yresidual != 1.0;
       bool shouldBlur = baton->blurSigma != 0.0;
-      bool shouldSharpen = baton->sharpenRadius != 0;
+      bool shouldSharpen = baton->sharpenSigma != 0.0;
       bool shouldThreshold = baton->threshold != 0;
       bool shouldPremultiplyAlpha = HasAlpha(image) &&
         (shouldAffineTransform || shouldBlur || shouldSharpen || hasOverlay);
@@ -598,7 +598,7 @@ class PipelineWorker : public AsyncWorker {
 
       // Sharpen
       if (shouldSharpen) {
-        image = Sharpen(image, baton->sharpenRadius, baton->sharpenFlat, baton->sharpenJagged);
+        image = Sharpen(image, baton->sharpenSigma, baton->sharpenFlat, baton->sharpenJagged);
       }
 
       // Composite with overlay, if present
@@ -1053,7 +1053,7 @@ NAN_METHOD(pipeline) {
   baton->flatten = attrAs<bool>(options, "flatten");
   baton->negate = attrAs<bool>(options, "negate");
   baton->blurSigma = attrAs<double>(options, "blurSigma");
-  baton->sharpenRadius = attrAs<int32_t>(options, "sharpenRadius");
+  baton->sharpenSigma = attrAs<double>(options, "sharpenSigma");
   baton->sharpenFlat = attrAs<double>(options, "sharpenFlat");
   baton->sharpenJagged = attrAs<double>(options, "sharpenJagged");
   baton->threshold = attrAs<int32_t>(options, "threshold");
diff --git a/src/pipeline.h b/src/pipeline.h
index d7ff6c03..49ca6d39 100644
--- a/src/pipeline.h
+++ b/src/pipeline.h
@@ -51,7 +51,7 @@ struct PipelineBaton {
   bool flatten;
   bool negate;
   double blurSigma;
-  int sharpenRadius;
+  double sharpenSigma;
   double sharpenFlat;
   double sharpenJagged;
   int threshold;
@@ -104,7 +104,7 @@ struct PipelineBaton {
     flatten(false),
     negate(false),
     blurSigma(0.0),
-    sharpenRadius(0),
+    sharpenSigma(0.0),
     sharpenFlat(1.0),
     sharpenJagged(2.0),
     threshold(0),
diff --git a/test/unit/sharpen.js b/test/unit/sharpen.js
index 22c75e24..244059de 100644
--- a/test/unit/sharpen.js
+++ b/test/unit/sharpen.js
@@ -7,10 +7,10 @@ var fixtures = require('../fixtures');
 
 describe('Sharpen', function() {
 
-  it('specific radius 10', function(done) {
+  it('specific radius 10 (sigma 6)', function(done) {
     sharp(fixtures.inputJpg)
       .resize(320, 240)
-      .sharpen(10)
+      .sharpen(6)
       .toBuffer(function(err, data, info) {
         assert.strictEqual('jpeg', info.format);
         assert.strictEqual(320, info.width);
@@ -19,10 +19,10 @@ describe('Sharpen', function() {
       });
   });
 
-  it('specific radius 3 and levels 0.5, 2.5', function(done) {
+  it('specific radius 3 (sigma 1.5) and levels 0.5, 2.5', function(done) {
     sharp(fixtures.inputJpg)
       .resize(320, 240)
-      .sharpen(3, 0.5, 2.5)
+      .sharpen(1.5, 0.5, 2.5)
       .toBuffer(function(err, data, info) {
         assert.strictEqual('jpeg', info.format);
         assert.strictEqual(320, info.width);
@@ -31,10 +31,10 @@ describe('Sharpen', function() {
       });
   });
 
-  it('specific radius 5 and levels 2, 4', function(done) {
+  it('specific radius 5 (sigma 3.5) and levels 2, 4', function(done) {
     sharp(fixtures.inputJpg)
       .resize(320, 240)
-      .sharpen(5, 2, 4)
+      .sharpen(3.5, 2, 4)
       .toBuffer(function(err, data, info) {
         assert.strictEqual('jpeg', info.format);
         assert.strictEqual(320, info.width);
@@ -55,9 +55,9 @@ describe('Sharpen', function() {
       });
   });
 
-  it('invalid radius', function() {
+  it('invalid sigma', function() {
     assert.throws(function() {
-      sharp(fixtures.inputJpg).sharpen(1.5);
+      sharp(fixtures.inputJpg).sharpen(-1.5);
     });
   });