Why does rim width matter?

Rim width affects tyre shape, contact patch, rolling resistance and handling. A rim that is too narrow causes a bulging tyre profile; too wide and the tyre becomes stretched and prone to burping.

What is the Rule of 105?

The Rule of 105, coined by Josh Poertner (formerly Zipp, now Silca) from 2001 wind-tunnel work, is an aerodynamics rule: the external (widest) rim width should be at least 105% of the measured tyre width so airflow leaving the tyre re-attaches to the rim and is smoothed, lowering drag. For a 25mm-wide tyre you want a rim at least 26.25mm wide externally. It is about aero efficiency, not fit safety — that is a separate ETRTO compatibility question (tyre 1.45–2.0× the internal rim width).

Are wider rims better?

Wider rims provide better tyre support, a rounder tyre profile, and improved cornering grip. They also allow lower tyre pressures without sidewall collapse. The trend is towards wider rims across all disciplines.

Can I put narrow tyres on wide rims?

There are safety limits. A tyre too narrow for the rim is poorly supported and prone to burping, and the exposed rim can be damaged on impacts. Per ETRTO and Schwalbe compatibility charts, tyre width should be at least 1.45× the internal rim width (the safe envelope is 1.45–2.0×). So a 25mm internal rim wants a tyre of at least ~36mm, and a 19mm internal rim at least ~28mm.

What about hookless rims?

Hookless rims require specific tyre compatibility (check manufacturer lists). They typically require slightly wider tyres relative to rim width and have specific pressure limits.

BikeCalcs

Rim Width Calculator — Optimal Rim for Your Tyres

Find the optimal internal rim width for your tyre size and riding style.

Tyre Width

Nominal (labelled) tyre width in mm, e.g. 28 for 700x28C, 40 for a 40mm gravel tyre, 60 for a 2.35in MTB tyre.

Riding TypeDiscipline sets the target rim-to-tyre ratio (road ~65%, gravel ~55%, MTB ~47%).

How We Calculate This

Rim Width Calculation

Optimal internal width = Tyre width × Optimal ratio. The ratio FALLS as tyres get wider.
Race: 62% ratio (slightly narrower rim, aero-led)
Road: 65% ratio (balanced — 28mm tyre → ~18mm internal)
Gravel: 55% ratio (40mm tyre → ~22mm internal, per ETRTO 21–25mm)
MTB: 47% ratio (tyre ≈ 2.0–2.3× internal rim; 60mm tyre → ~28mm internal)
External width = Internal + ~5mm (rim wall thickness)

Frequently Asked Questions

Related Calculators

Tyre Pressure Calculator

Calculate optimal tyre pressure based on rider weight and tyre width.

Tyre Size Converter

Convert between ETRTO, French and imperial tyre size designations.

Wheel Circumference Calculator

Calculate wheel circumference from tyre size for computer setup.

Spoke Length Calculator

Calculate spoke lengths for wheel building from hub and rim dimensions.

Spoke Count Calculator

Determine the right spoke count for your riding style and weight.

Last updated: February 2026

All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.