Radiator Cap Pressure Calculator
A pressure cap is what keeps coolant from boiling. See what your cap is worth in degrees, and what a weak one costs you.
Cap and coolant
- Never fit a cap rated higher than the system was designed for. Hoses, the heater core and the radiator all have pressure limits.
- Roughly 3F of boiling point per psi is a good working approximation across normal cooling system pressures.
- Never open a hot cooling system. Pressurised coolant above its atmospheric boiling point flashes to steam instantly.
Never open a hot cooling system. Coolant held above its atmospheric boiling point by system pressure will flash to steam the moment the cap is released. Let the engine cool fully before opening anything.
Why the system is pressurised
A 50/50 coolant mix boils at 226F at sea level, which is not much above where an engine runs. Under load, coolant leaving the head is hotter than the gauge suggests, and local hot spots around the exhaust valves are hotter still.
Pressure fixes that. Every psi raises the boiling point by roughly three degrees, so a 16 psi cap adds nearly 50 degrees of margin and takes a 226F mixture up past 270F.
A weak cap looks like a bad radiator
Caps fail quietly. The rubber seal hardens and the spring weakens, and it starts releasing at 8 psi instead of 16. Nothing leaks and nothing looks wrong, but you have lost 25 degrees of boiling margin.
The symptom is overheating under load that a new radiator, thermostat and fresh coolant do not fix. A cap costs a few dollars and any parts store can pressure test the old one.
Do not fit a higher rated cap
More pressure means more margin, which tempts people to fit a stronger cap to cure an overheating problem. The system was designed around a specific pressure, and hoses, the heater core and the radiator seams all have limits. Raising the cap rating usually just moves the failure somewhere more expensive.
Frequently asked questions
How much does a radiator cap raise the boiling point?
About 3F for every psi. A 16 psi cap on a 50/50 mix takes the boiling point from roughly 226F to about 274F, which is nearly 50 degrees of extra margin.
Can a bad radiator cap cause overheating?
Yes, and it is one of the most commonly missed causes. A cap releasing early lowers the boiling point, so coolant boils under load, and steam pockets do not carry heat. Everything else in the system can test perfectly.
Can I fit a higher pressure cap?
You should not. The rest of the system, including hoses, the heater core and the radiator itself, is designed for a specific pressure. A higher rated cap raises boiling margin at the cost of blowing something more expensive.
Why is my coolant boiling over after I shut the engine off?
Heat soak. With the water pump stopped, heat from the block migrates into stationary coolant and the temperature spikes. A healthy cap and expansion tank handle it. A weak cap releases early and pushes coolant out onto the ground.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.