From the Ideal Gas Law we have PV = nRT. Assuming a constant volume (a reasonable assumption in most conditions except for maybe extremely low pressures), the equation can be re-arranged to get variables on one side, constants on the other: P/T = nR/V. From there, P1/T1 = P2/T2 or
P1(T2/T1) = P2
Just convert temperatures to your favorite absolute scale and find the ratio. The pressures in the table correspond to my front and rear mountain tires and front and rear road tires, but the percentage is what really matters.
For a general rule of thumb, if it’s 100 degrees outside and your pumping up tires inside, decrease pressure by *about 5%. If it’s 45 outside, increase by 5%. If it’s 15, increase by 10%. Under “normal” riding conditions the pressure difference isn’t really noticeable, but in 15 degree weather, my tires felt a bit squishy.
While I was calculating all this, I started thinking about shock pressure. Shocks are different in that a change in temperature results in a constant pressure process rather than a constant volume process. Without knowing the volume of the air spring or the number of moles of air, the change in volume can’t be calculated. But, assuming you want the same sag level regardless of temperature, the volume is essentially a forced constant, so the calculations above still apply in regards to adjusting the pressure according to outside temperature.
*Technically, if you expect a 5% drop you should increase by 5.263%. However, indoor and outdoor temperatures fluctuate, and most pressure gauges don’t have this level of precision anyway. Even getting it within 4 – 6% takes a bit of effort, especially at low pressures.
