diff --git a/tests/unit/spinner/spinner_options.js b/tests/unit/spinner/spinner_options.js
index 647455fe5..48bdf96a9 100644
--- a/tests/unit/spinner/spinner_options.js
+++ b/tests/unit/spinner/spinner_options.js
@@ -2,6 +2,71 @@
 
 module( "spinner: options" );
 
+test( "incremental, false", function() {
+	expect( 100 );
+
+	var i, diff,
+		prev = 0,
+		element = $( "#spin" ).val( prev ).spinner({
+			incremental: false,
+			spin: function( event, ui ) {
+				equal( ui.value - prev, 1 );
+				prev = ui.value;
+			}
+		});
+
+	for ( i = 0; i < 100; i++ ) {
+		element.simulate( "keydown", { keyCode: $.ui.keyCode.UP } );
+	}
+});
+
+test( "incremental, true", function() {
+	expect( 100 );
+
+	function fill( num, val ) {
+		return $.map( new Array( num ), function() {
+			return val;
+		});
+	}
+
+	var i, diff,
+		prev = 0,
+		expected = [].concat( fill( 18, 1 ), fill( 37, 2 ), fill( 14, 3 ),
+			fill( 9, 4 ), fill( 6, 5 ), fill( 5, 6 ), fill ( 5, 7 ),
+			fill( 4, 8 ), fill( 2, 9 ) ),
+		element = $( "#spin" ).val( prev ).spinner({
+			incremental: true,
+			spin: function( event, ui ) {
+				equal( ui.value - prev, expected[ i ] );
+				prev = ui.value;
+			}
+		});
+
+	for ( i = 0; i < 100; i++ ) {
+		element.simulate( "keydown", { keyCode: $.ui.keyCode.UP } );
+	}
+});
+
+test( "incremental, function", function() {
+	expect( 100 );
+
+	var i,
+		prev = 0,
+		element = $( "#spin" ).val( prev ).spinner({
+			incremental: function( i ) {
+				return i;
+			},
+			spin: function( event, ui ) {
+				equal( ui.value - prev, i + 1 );
+				prev = ui.value;
+			}
+		});
+
+	for ( i = 0; i < 100; i++ ) {
+		element.simulate( "keydown", { keyCode: $.ui.keyCode.UP } );
+	}
+});
+
 test( "numberFormat, number", function() {
 	expect( 2 );
 	var element = $( "#spin" ).val( 0 ).spinner({ numberFormat: "n" });
@@ -26,44 +91,6 @@ test( "numberFormat, currency", function() {
 	equal( element.val(), "$1.00", "formatted after step" );
 });
 
-/* TODO figure out how to test this properly
-test("incremental - false (default)", function() {
-	var el = $("#spin").spinner({ incremental:false });
-
-	for ( var i = 1 ; i<=120 ; i++ ) {
-		el.simulate("keydown",{keyCode:$.ui.keyCode.UP});
-	}
-	el.simulate("keyup",{keyCode:$.ui.keyCode.UP});
-
-	equals(el.val(), 120, "incremental false - keydown 120 times");
-
-	for ( var i = 1 ; i<=210 ; i++ ) {
-		el.simulate("keydown",{keyCode:$.ui.keyCode.DOWN});
-	}
-	el.simulate("keyup",{keyCode:$.ui.keyCode.DOWN});
-
-	equals(el.val(), -90, "incremental false - keydown 210 times");
-});
-
-test("incremental - true (default)", function() {
-	var el = $("#spin").spinner();
-
-	for ( var i = 1 ; i<=120 ; i++ ) {
-		el.simulate("keydown",{keyCode:$.ui.keyCode.UP});
-	}
-	el.simulate("keyup",{keyCode:$.ui.keyCode.UP});
-
-	equals(el.val(), 300, "incremental true - keydown 120 times (100+20*10)");
-
-	for ( var i = 1 ; i<=210 ; i++ ) {
-		el.simulate("keydown",{keyCode:$.ui.keyCode.DOWN});
-	}
-	el.simulate("keyup",{keyCode:$.ui.keyCode.DOWN});
-
-	equals(el.val(), -1800, "incremental true - keydown 210 times (300-100-100*10-10*100)");
-});
-*/
-
 test( "max", function() {
 	expect( 3 );
 	var element = $( "#spin" ).val( 1000 ).spinner({ max: 100 });
diff --git a/ui/jquery.ui.spinner.js b/ui/jquery.ui.spinner.js
index 97ee20e54..a7fb30cc3 100644
--- a/ui/jquery.ui.spinner.js
+++ b/ui/jquery.ui.spinner.js
@@ -233,9 +233,15 @@ $.widget( "ui.spinner", {
 	},
 
 	_increment: function( i ) {
-		return this.options.incremental ?
-			Math.floor( i*i*i/50000 - i*i/500 + 17*i/200 + 1 ) :
-			1;
+		var incremental = this.options.incremental;
+
+		if ( incremental ) {
+			return $.isFunction( incremental ) ?
+				incremental( i ) :
+				Math.floor( i*i*i/50000 - i*i/500 + 17*i/200 + 1 );
+		}
+
+		return 1;
 	},
 
 	_precision: function( num ) {