Ideal Arm Size Calculator
Wrist size is a rough proxy for skeletal frame. A well-known old-school formula scales an "ideal" arm from it.
About this tool
The classic formula, popularised by strength writers such as John McCallum decades ago, sets an ideal flexed upper-arm (bicep) circumference at about 2.5 times wrist circumference. It is a heuristic built around average proportions, not a biological law, but it gives a frame-adjusted target rather than a flat number that ignores how big your skeleton is.
A visually balanced arm also keeps the forearm close to about 80% of the upper-arm circumference. An arm that is much bigger than the forearm and wrist can look less proportioned even at a large absolute size.
None of this predicts how big your arms could get naturally - the Muscle Potential Calculator handles that from height, wrist and ankle size. This tool is purely about proportion.
How to read your result
Below the classic ratio.
Classic, balanced ratio.
Above average.
Well above the classic ratio - check forearm balance.
Frequently asked questions
Where does the 2.5x figure come from?
Is a bigger ratio always better?
Does this account for body fat?
This tool is for information and education only and is not medical advice. For adults 18 and over. Read the full disclaimer.
Related tools
Read next
Body Proportions Guide: Ratios, Heads Tall, Leg Length, Arm Span and What They Mean
The main body proportions people measure, typical ranges for each, how to measure them, and what they mean for clothing, liftin…
Natural Muscle Limit: How Much Muscle Can You Realistically Build?
Realistic muscle gain rates by year of training, what FFMI and the Casey Butt formula estimate, what limits your ceiling, and h…