diff --git a/API.md b/API.md
index b5bb4763..332f2947 100755
--- a/API.md
+++ b/API.md
@@ -252,6 +252,7 @@ The reference names can have one of the following prefixes:
 
 The formula syntax also supports built-in functions:
 - `if(condition, then, otherwise)` - returns `then` when `condition` is truthy, otherwise `otherwise`.
+- `length(item)` - reutrn the length of an array or string, the number of keys of an object, otherwise `null`.
 - `msg(code)` - embeds another error code message.
 - `number(value)` - cast value to a number.
 
diff --git a/lib/template.js b/lib/template.js
index 440e4a10..3858ad17 100755
--- a/lib/template.js
+++ b/lib/template.js
@@ -385,6 +385,23 @@ internals.functions = {
         return condition ? then : otherwise;
     },
 
+    length(item) {
+
+        if (typeof item === 'string') {
+            return item.length;
+        }
+
+        if (!item || typeof item !== 'object') {
+            return null;
+        }
+
+        if (Array.isArray(item)) {
+            return item.length;
+        }
+
+        return Object.keys(item).length;
+    },
+
     msg(code) {
 
         const [value, state, prefs, local, options] = this;
diff --git a/test/template.js b/test/template.js
index 1e361910..9c0ca1e4 100755
--- a/test/template.js
+++ b/test/template.js
@@ -201,5 +201,71 @@ describe('Template', () => {
                 Helper.validate(schema, { context: { x: {} } }, [[4, false, '"value" must be [{number(1) + number(true) + number(false) + number("1") + number($x)}]']]);
             });
         });
+
+        describe('length()', () => {
+
+            it('calculates object size', () => {
+
+                const schema = Joi.object({
+                    a: Joi.array().length(Joi.x('{length(b)}')),
+                    b: Joi.object()
+                });
+
+                Helper.validate(schema, [
+                    [{ a: [1, 2], b: { a: true, b: true } }, true],
+                    [{ a: [1, 2, 3], b: { a: true, b: true } }, false, '"a" must contain {length(b)} items']
+                ]);
+            });
+
+            it('calcualtes array size', () => {
+
+                const schema = Joi.object({
+                    a: Joi.array().length(Joi.x('{length(b)}')),
+                    b: Joi.array()
+                });
+
+                Helper.validate(schema, [
+                    [{ a: [1, 2], b: [2, 3] }, true],
+                    [{ a: [1, 2, 3], b: [1] }, false, '"a" must contain {length(b)} items']
+                ]);
+            });
+
+            it('handles null', () => {
+
+                const schema = Joi.object({
+                    a: Joi.array().length(Joi.x('{length(b)}')),
+                    b: Joi.array().allow(null)
+                });
+
+                Helper.validate(schema, [
+                    [{ a: [1], b: null }, false, '"a" limit references "{length(b)}" which must be a positive integer']
+                ]);
+            });
+
+            it('handles strings', () => {
+
+                const schema = Joi.object({
+                    a: Joi.array().length(Joi.x('{length(b)}')),
+                    b: Joi.string()
+                });
+
+                Helper.validate(schema, [
+                    [{ a: [1], b: 'x' }, true],
+                    [{ a: [1], b: 'xx' }, false, '"a" must contain {length(b)} items']
+                ]);
+            });
+
+            it('handles items without length', () => {
+
+                const schema = Joi.object({
+                    a: Joi.array().length(Joi.x('{length(b)}')),
+                    b: Joi.number()
+                });
+
+                Helper.validate(schema, [
+                    [{ a: [1], b: 1 }, false, '"a" limit references "{length(b)}" which must be a positive integer']
+                ]);
+            });
+        });
     });
 });