diff --git a/readme.md b/readme.md
index 16973c6..d37a46e 100644
--- a/readme.md
+++ b/readme.md
@@ -4069,6 +4069,7 @@ Tubing and sleeving. The internal diameter can be forced to stretch it over some
 ### Functions
 | Function | Description |
 |:--- |:--- |
+| `tubing_ir(type)` | Inside radius |
 | `tubing_or(type)` | Outside radius |
 
 ### Modules
diff --git a/vitamins/tubing.scad b/vitamins/tubing.scad
index 3679780..6419e6e 100644
--- a/vitamins/tubing.scad
+++ b/vitamins/tubing.scad
@@ -29,6 +29,7 @@ function tubing_id(type)       = type[3]; //! Inside diameter
 function tubing_colour(type)   = type[4]; //! Colour
 
 function tubing_or(type)        = tubing_od(type) / 2; //! Outside radius
+function tubing_ir(type)        = tubing_id(type) / 2; //! Inside radius
 
 module tubing(type, length = 15, forced_id = 0, center = true) { //! Draw specified tubing with optional forced internal diameter
     original_od = tubing_od(type);
@@ -50,4 +51,3 @@ module tubing(type, length = 15, forced_id = 0, center = true) { //! Draw specif
                     circle(d = id);
                 }
 }
-