diff --git a/modules/vec2.lua b/modules/vec2.lua
index 01a0b97..030937f 100644
--- a/modules/vec2.lua
+++ b/modules/vec2.lua
@@ -321,6 +321,14 @@ function vec2.to_polar(a)
 	return radius, theta
 end
 
+-- Convert vec2 to vec3.
+-- @tparam vec2 a Vector to convert.
+-- @tparam number the new z component, or nil for 0
+-- @treturn vec3 Converted vector
+function vec2.to_vec3(a, z)
+	return vec3(a.x, a.y, z or 0)
+end
+
 --- Return a formatted string.
 -- @tparam vec2 a Vector to be turned into a string
 -- @treturn string formatted
diff --git a/spec/vec2_spec.lua b/spec/vec2_spec.lua
index 8b826ad..4bd0305 100644
--- a/spec/vec2_spec.lua
+++ b/spec/vec2_spec.lua
@@ -189,7 +189,16 @@ describe("vec2:", function()
 		assert.is.equal("(+0.000,+0.000)", b)
 	end)
 
-	-- Do this last, to insulate tests from accidental state contamination
+	-- Do vec3 tests last, to insulate tests from accidental state contamination
+	it("converts a 2-vector to a 3-vector", function()
+		local vec3 = require "modules.vec3"
+		local a = vec2(1,2)
+		local b = a:to_vec3()
+		local c = a:to_vec3(3)
+		assert.is.equal(b, vec3(1,2,0))
+		assert.is.equal(c, vec3(1,2,3))
+	end)
+
 	it("converts a vec3 to vec2 using the constructor", function()
 		local vec3 = require "modules.vec3"
 		local a = vec2(3, 5)