SILCA Pressure Calculator
Find your exact aerodynamic and rolling resistance impedance breakpoint using the pro-tour methodology developed by SILCA. Covers road, gravel, MTB, and cyclocross — clincher, tubeless, and tubular. Also see the road bike PSI guide for full width tables.
Real-World Example
65 kg Rider · 47mm Gravel Tire — SILCA Calculator Output
Inputs: 65 kg total system weight (rider + bike + gear) · 47mm measured tire width · high-performance tubeless · gravel weight distribution · moderate group ride pace
| Wheel · Surface | Front PSI | Rear PSI |
|---|---|---|
| 650B (27.5") · Cat 1 Gravel — hardpack / light gravel | 31.7 | 33.2 |
| 650B (27.5") · Cat 2 Gravel — moderate gravel | 28.7 | 30.0 |
| 700C (29") · Cat 1 Gravel — hardpack / light gravel | 30.0 | 31.4 |
| 700C (29") · Cat 2 Gravel — moderate gravel | 27.2 | 28.4 |
Values computed with the SILCA impedance algorithm. Cat 1 = hardpack dirt / light gravel. Cat 2 = moderate mixed gravel. Enter your exact inputs below for a personalised result.
Typical Scenarios
SILCA Tire Pressure: Common Examples
Smooth tarmac vs rough pavement
A 175 lb (79 kg) total system weight running high-performance tubeless tires with a measured width of 30mm on new smooth tarmac should run 73.5 PSI rear / 71.7 PSI front. Transitioning to rough, worn pavement, the algorithm drops to 69.6 PSI rear / 67.9 PSI front — a 3.9 PSI reduction that prevents vibration absorption from wasting watts at speed.
Stated vs measured width error
A rider inputs their tire's stated width of 28mm and the calculator returns 82 PSI rear. But their tire, mounted on a modern 25mm internal-width rim, actually measures 31mm. The correct input gives 70 PSI rear — a 12 PSI difference that completely changes rolling resistance and comfort. Always measure; never guess.
Gravel event scenario
A 175 lb (79 kg) system on a 40mm measured gravel tire (650B wheel) targeting hardpack dirt should run 44.9 PSI rear / 42.9 PSI front (cat1-gravel, high-performance tubeless). Moving to moderate mixed gravel drops both by 4.1 PSI to 40.8 PSI rear / 39.0 PSI front. Switching to a mid-range tubeless casing on hardpack reduces output by a further 1.3 PSI to 43.6 PSI rear / 41.7 PSI front.
Quick Reference
SILCA PSI by Surface Category
175 lb (79.4 kg) total system weight · 30mm measured width · 700C · road weight distribution · high-performance tubeless · moderate group ride pace
| Surface Condition | SILCA Category | Front PSI | Rear PSI |
|---|---|---|---|
| New, Smooth Tarmac | Cat 1 | 71.7 PSI | 73.5 PSI |
| Rough / Worn Pavement | Cat 2 | 67.9 PSI | 69.6 PSI |
| Hardpack Dirt / Light Gravel | Cat 3 | 65.0 PSI | 66.7 PSI |
| Moderate Gravel | Cat 4 | 59.1 PSI | 60.6 PSI |
| Deep / Chunky Gravel | Cat 5 | 48.0 PSI | 49.2 PSI |
Use the calculator above for your exact weight and measured tire width. When in doubt between two surface categories, choose the rougher one.
Step-by-Step
How to Use This Calculator
- 1
Calculate total system weight
Weigh yourself fully kitted — helmet, shoes, hydration pack — and add the exact weight of your bicycle. SILCA uses total system weight, not body weight alone. A 155 lb rider on a 17 lb bike = 172 lb system weight.
- 2
Determine measured tire width
Do not use the number printed on the tire sidewall. Use digital calipers to measure the actual inflated width. A '28c' tire on a modern 25mm internal-width rim balloons to 30–31mm. Inputting stated width instead of measured width is the single most common source of SILCA miscalculation.
- 3
Select surface condition
Choose the SILCA surface category that best matches your ride. Smooth tarmac (Cat 1) runs the highest PSI; chunky gravel (Cat 5) the lowest. When in doubt between two categories, choose the rougher one — erring toward lower pressure protects against suspension losses.
- 4
Select tire casing type
High-performance casings (supple 320 TPI cotton) deform more easily and need slightly higher pressure to prevent collapse. Stiff puncture-resistant casings require slightly less pressure to achieve the same contact patch shape.
- 5
Select wheel diameter
700c is standard road; 650b is common on gravel and smaller riders; 26" covers older MTB and some touring bikes. Smaller diameter wheels need marginally higher pressure to achieve the same air volume and casing drop.
- 6
Calculate and note your front/rear split
The SILCA algorithm automatically applies a front/rear weight distribution bias — typically 2–4 PSI lower front. Note both numbers and inflate your tires separately. Never inflate both to the rear PSI figure.
Methodology
How the SILCA Algorithm Works
Core formula
Optimal PSI = (System Weight × Width/Volume Constant)
− Surface Impedance Penalty
− Casing Stiffness Modifier
Front PSI = Rear PSI × Front/Rear Weight Distribution Ratio
Variable definitions
- Surface Impedance Penalty: As the road gets rougher, the algorithm lowers PSI to let the tire absorb bumps rather than transferring vibration energy into the rider. Deep/chunky gravel (Cat 5) output is approximately 33% lower than smooth new tarmac (Cat 1) at the same system weight and tire width.
- Width/Volume Constant: Air volume increases with the square of tire width, so a 30mm tire requires drastically less pressure than a 28mm tire to achieve the same casing tension. This is why using measured width — not stated width — is critical.
- Casing Stiffness Modifier: Supple, high-TPI tires (cotton or 320+ TPI) use coefficient 1.0 as the baseline. Mid-range tubeless reduces output by ~3%, mid-range butyl tube by ~6%, and puncture-resistant casings by ~9% — a practical range of 2–7 PSI between the most and least supple options at typical road pressures.
Worked example
180 lb (81.6 kg) system weight · 28mm measured tire · 700C · road weight distribution · high-performance tubeless · moderate group ride pace
New smooth tarmac (baseline) = 82.3 PSI rear / 80.3 PSI front
Worn pavement surface factor = 78.0 PSI rear / 76.1 PSI front (−5.2%)
Worn pave + mid-range butyl tube = 73.3 PSI rear / 71.5 PSI front (casing −6%)
Road distribution (48/52 split) → front is 97.5% of rear in all rows above
FAQ