Pressure is how you steer temperature
A tyre grips inside a temperature window, and its pressure when hot decides how it builds and holds that temperature. This page is the whole method the calculator implements, in the order you use it on the day. Nothing here needs the app; the app just does the arithmetic faster than a cold thumb.
Start from where the tyre should end up
The number that matters is the pressure your tyre should show when fully worked: the hot target. For some car and tyre pairings a published figure exists from the manufacturer or a race series, and GripCalc carries 27 of those verbatim. For the rest it estimates from the vehicle's class and weight distribution and labels the result an estimate. Racing typically targets one to two psi more than a track day, for response at the cost of wear.
The gas law does the rest
Air pressure rises with temperature in fixed proportion on absolute scales: P₁/T₁ = P₂/T₂. Knowing the target pressure at working temperature and the ambient temperature this morning, the cold pressure follows, corner by corner. The corners differ because a circuit works them differently; every layout in Circuits carries starting figures for how much hotter each corner runs. Set the pressures with the tyres genuinely cold: a car that has driven twenty minutes to the airline has already warmed them.
Hot pressures straight off the circuit
After the first session, check hot pressures immediately. Above target: you started too high or the corner is overworking; bleed down toward the target. Below: add air. This alone puts you ahead of most of the paddock. The next step puts you ahead of the rest.
Three points per tyre, within a minute
A probe pyrometer reads the tread's core temperature at inner, middle and outer zones; the surface cools within seconds, so work fast and in the same order every time. The pattern across each tread is the diagnosis: crown hotter than shoulders reads as overinflation, shoulders hotter than crown as underinflation, and an inner-to-outer slope speaks to camber for that corner. The calculator's analysis encodes these rules, scores each corner against the tyre's window, and keeps the session so the next one starts from measurement instead of a model.
When the right answer is the handbook
On the road none of the above applies: tyres never reach circuit temperatures, and the manufacturer has already published the correct cold pressures for your exact car on the placard. The only physics worth adding is compensation: a pressure set in a warm garage reads low on a cold morning, and road mode restates your placard figure for the day. A calculator should know when not to calculate.