Methodology and Sources

Our principles

• Formulas in plain sight. No calculator is a black box: each page explains how it works, and this page sums it all up.
• Sourced data. Standards, manufacturer specs, studies and market figures come with a link. If we can't verify a number, we don't publish it.
• Safety first. Safety limits (maximum pressure for hookless rims and for tires) always override the calculation.
• Independence. Nobody pays us to be featured or recommended. Some store links are affiliate links, and we say so (see our affiliate disclosure).
• Corrections. If you find a mistake, let us know: we fix it and update the page date.

Tire pressure

We start from Frank Berto's rule: a tire is properly inflated when it compresses by a constant fraction of its height under load. From there we built our own model, calibrated against the pressures riders use today on the road, on gravel and on mountain bikes:

Base pressure (psi) = 165 × weight on the wheel (kg) ÷ width (mm)^1.4 × √(700c diameter ÷ your wheel diameter)

Weight distribution ranges from 40% on the front wheel (city bike) to 48% (triathlon). On top of the base we apply an adjustment for the type of riding (+12-18% for mountain, city and e-bikes), the surface (from +6% on the velodrome to −22% on technical singletrack), the tube (+4% with butyl), the casing, your preference and wet roads (−5%). Actual mounted width is estimated from the internal rim width (about 0.4 mm for every mm of rim width above the reference rim). Then the limits kick in: 5 bar (73 psi) on hookless rims (ETRTO and ISO 4210-2), the tire's maximum, and a minimum so you don't bottom out on the rim. Open the calculator.

Frame size, saddle height and cranks

• Saddle height (center of the bottom bracket to the top of the saddle): the average of LeMond (inseam × 0.883, corrected for crank length) and the 109% method of Hamley and Thomas (1967), with a ±1 cm (±0.4 in) window to fine-tune for a knee angle of 25-35° (Holmes et al., 1994).
• Frame size: generic size charts by height, checked against several brands' charts, plus the classic sizing by inseam (× 0.665 for road, × 0.226 in inches for mountain bikes) and your leg-to-height ratio to choose between two sizes.
• Cranks: the classic chart by inseam and today's trend toward shorter cranks.

Gearing, chain and spokes

• Rollout (m) = chainring ÷ cog × wheel circumference. Circumference ≈ π × (ISO bead seat diameter + 2 × 0.97 × tire width).
• Speed (km/h) = rollout × cadence × 60 ÷ 1,000. Gain ratio (Sheldon Brown) = wheel radius ÷ crank length × chainring ÷ cog.
• Chain (inches) = 2 × chainstay + (big chainring + big cog) ÷ 4 + 1, rounded to an even number of links.
• Spokes: the standard geometric formula based on ERD, flange diameter, center-to-flange distance and number of crosses.

Power, speed and climbs

We use the model of Martin et al. (1998, Journal of Applied Biomechanics): power goes into overcoming air resistance, rolling resistance and gravity, plus drivetrain losses (2.5%). Air density comes from altitude and temperature. Aerodynamic (CdA) and rolling resistance (Crr) coefficients are typical values for each riding position and surface. For climbs we solve speed segment by segment and calculate VAM (vertical meters climbed per hour). Each climb's difficulty uses the FIETS index.

FTP, zones and calories

Power zones follow Andrew Coggan's seven-zone model; heart rate zones use the percentage of maximum heart rate, heart rate reserve (Karvonen) and threshold methods. Each calculator cites its sources and explains its limits.

Limits

These are estimates to help you decide, not measurements. A professional bike fit, a power meter or a scale gives you real data that beats any calculation. Always follow your bike, wheel and tire manufacturer's specifications.