Free automotive calculators and tools — built by mechanics, for mechanics. No signup. No ads inside the tools. No data collection. Everything runs in your browser. From engine builds to brake jobs to pre-purchase inspections to road trip planning, the math you actually need to be the mechanic.
Performance & Tuning
10 toolsEngine specs, power conversion, dyno correction, boost math, and fuel system sizing.
Torque & Horsepower Calculator
HP, torque, and RPM relationship at any engine speed.
Engine Displacement Calculator
Bore and stroke to CC, cubic inches, and liters.
Dyno Correction Calculator
SAE J1349 and DIN correction factors for accurate power.
Power Converter
HP, kW, PS, BHP, lb-ft, Nm two-way conversion.
Power to Weight Ratio
hp per pound, hp per ton, and W per kg benchmark.
Supercharger Boost Calculator
Pulley ratio, SC RPM, and predicted boost pressure.
Turbo Boost Calculator
Intake manifold pressure, density ratio, and boost target.
Compression Ratio Calculator
Static CR from bore, stroke, chamber, and piston volume.
Fuel Injector Size Calculator
Required injector flow rate for any target horsepower.
AFR Calculator
Air-fuel ratio and lambda conversion for any fuel.
Vehicle Dynamics
4 toolsDrivetrain, suspension, wheel fitment, and towing capacity math.
Vehicle Speed Calculator
MPH from RPM, gear ratio, and tire size.
Wheel Fitment Calculator
Offset, bolt pattern, hub bore, and rolling diameter check.
Corner Weight Calculator
Front/rear bias, cross weight, and chassis setup.
Towing & Payload Calculator
GVWR, GCWR, tongue weight, and axle load check.
Maintenance & MPG
5 toolsBrake life, tire pressure, fuel economy, and cooling system math.
Brake Pad Wear Calculator
Remaining pad life and estimated replacement date.
Tire Pressure Temperature
Cold-to-hot pressure adjustment for any climate.
Actual MPG Calculator
Measure real world fuel economy from your fill ups.
MPG vs Speed Calculator
See how cruising speed shapes your fuel economy curve.
Coolant Mix Calculator
Right antifreeze ratio for your system and climate.
Battery & Electrical
3 toolsBattery sizing, state of health diagnosis, and 12V wiring.
Battery Group Size Finder
Correct BCI group, CCA, and terminal layout for your vehicle.
Battery State of Health
Estimate remaining battery life from voltage, CCA, and age.
Wire Gauge Calculator
Correct AWG size for any 12V automotive circuit.
EV & Fuel
2 toolsElectric vehicle charging math and road trip fuel cost estimates.
EV Charging Calculator
Time, cost, and miles added per hour at any kW level.
Trip Cost Calculator
Estimate fuel cost for any road trip from distance and MPG.
Fluids
1 toolTwo-stroke premix math for outdoor power equipment.
Buying & Owning
3 toolsSmart math for buying a car, owning it well, and saving on long-term repair costs.
Used Car Inspection Checklist
60+ point pre-purchase inspection with severity scoring and verdict.
Auto Loan Calculator
Monthly payment, total interest, and full amortization schedule.
DIY Repair Savings Calculator
Compare shop vs DIY cost for 35+ common repairs with lifetime savings.
Why We Built These
Every one of these calculators exists because somewhere out there, a dealer service advisor, a parts counter clerk, or a sketchy aftermarket sales site gave someone bad math and an inflated bill. The numbers are not the hard part. Knowing what numbers to chase, and being able to do the math yourself before someone tells you what it should be, is the difference between getting taken and getting a fair deal. These tools are free, no signup, no ads in the calculators themselves, no data collection. You can be the mechanic.
Missing a Calculator You Need?
Have a tool you wish existed? Want a feature added to one of these 28? Send us a note and we will look at every message. These tools improve when the people using them tell us what is missing.