Free Auto Repair Calculators

38 free tools for working on your own vehicle. No sign up, no app, nothing to install.

Brakes 5

Brake Pad Life Calculator Measure your brake pads and find out how many miles are left Brake Rotor Thickness Calculator Measure your rotor against its discard spec and find out whether it can be machined or has to be replaced Stopping Distance Calculator How far it really takes to stop, split into reaction distance and braking distance, for any speed, road surface and tire condition Brake Fluid Boiling Point Calculator Brake fluid absorbs moisture and its boiling point falls Brake Bias Calculator Work out front to rear brake torque split from piston sizes and rotor diameters, and see what a big brake kit does to the balance

Tires and wheels 8

Tire Size Comparison Calculator Compare two tire sizes side by side Wheel Offset Calculator Compare wheel offset and backspacing between two setups Tire Tread Depth Calculator Measure your tread and find out how many miles are left before the tires are worn out, with the thresholds that matter for wet and snow Tire Load Index Calculator Decode the load index and speed rating on your sidewall, and check whether a set of tires is rated to carry your vehicle's weight Bolt Pattern Calculator Measure between studs and get the bolt circle in millimetres and inches Tire Pressure Temperature Calculator Cold weather drops tire pressure and trips the TPMS light Tire Rotation Pattern Guide The right rotation pattern depends on your drivetrain and tires Lug Nut Torque Calculator Find the right torque for your wheel studs in ft-lb and Nm, with the correct tightening pattern for your lug count and seat type

Engine 5

Compression Ratio Calculator Calculate static compression ratio from bore, stroke, chamber, gasket, deck and piston volume Engine Displacement Calculator Bore, stroke and cylinder count into cubic inches, cc and litres Volumetric Efficiency Calculator Work out the airflow your engine needs at a given rpm, or the volumetric efficiency implied by an airflow figure you already have Boost Horsepower Calculator Estimate what a given boost pressure adds to your engine, allowing for intercooling, supercharger drive loss and altitude Trap Speed Horsepower Calculator Estimate engine power from a quarter mile time slip

Fuel and air 2

Fuel Injector Size Calculator Work out the injector size your power target needs in lb/hr and cc/min, at a duty cycle that leaves real headroom Fuel Pump Size Calculator Find the pump flow your power target needs in litres per hour, with the margin real fuel systems require for pressure, voltage and age

Electrical 4

Battery CCA Calculator Work out the cold cranking amps your engine needs for your climate, and how much a cold morning and battery age take away from what you have Alternator Size Calculator Add up your electrical load and find out whether the alternator can keep up at idle and at cruise, or how big a replacement needs to be Parasitic Draw Calculator Measure the current your parked vehicle draws and find out how many days it takes to flatten the battery, plus how to track the fault down Wire Gauge Calculator Find the right copper wire gauge for any 12V or 24V circuit

Drivetrain 2

Gear Ratio Calculator Work out engine RPM at any road speed from your tire size, axle ratio and gear Speedometer Calibration Calculator Work out how far off your speedometer reads after a tire or gear change, and what it takes to correct it on cable or electronic setups

Cooling and HVAC 3

Coolant Mixture Calculator Work out how much antifreeze and distilled water your system needs Radiator Cap Pressure Calculator See what your pressure cap is worth in degrees of boiling point, and what a weak one costs you A/C Oil Charge Calculator Replacing an A/C part takes oil out with it

Suspension and steering 3

Spring Rate Converter Convert spring rates between lb/in, N/mm and kg/mm Toe Conversion Calculator Convert alignment toe between inches, millimetres, degrees and minutes for your tire size Sway Bar Stiffness Calculator Compare two anti-roll bars

Exhaust 2

Exhaust Pipe Diameter Calculator Size exhaust tubing for your power level without falling into the bigger-is-better trap Catalytic Converter Legality Checker Some states require a CARB compliant converter and fail an EPA one at inspection

Maintenance and workshop 4

Oil Change Interval Calculator The 3,000 mile rule is decades out of date Oil Viscosity Calculator What 5W-30 actually means, and which grade suits your climate and engine Maintenance Schedule Calculator Enter your mileage and age and see what is due now, what is coming up, and which time-based items your odometer will never reach Torque Conversion Calculator Convert between foot-pounds, newton-metres and inch-pounds
For guidance only

Every calculator here produces estimates worked out from the numbers you enter and typical figures for the conditions you pick. 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.