Unit Converters Calculators
13 free unit converters calculators — standards-based, with formulas, worked examples, and no sign-up.
Every conversion in this category runs through the quantity's SI base unit — pascal for pressure, m³/s for flow, watt for power, joule for energy, newton-meter for torque — using the factors published in NIST SP 811, the NIST Guide for the Use of the International System of Units, Appendix B. That single sourcing decision is what makes the converters trustworthy: bar, atm, kgf/cm², kWh, and the international foot family are exact by definition, while psi, BTU, horsepower, and their relatives carry NIST's published 7-significant-figure values, and each factor is cited in the tool.
Pick the dedicated tool when your quantity has one: pressure (psi, bar, kPa, atm, Torr), flow rate (CFM, GPM, L/s, m³/h), temperature (the one affine scale here — Celsius, Fahrenheit, Kelvin, and Rankine convert with offsets, not factors, and the tools guard the absolute-zero floor), torque (Nm, ft-lb, kgf·m), power (kW, HP both mechanical and metric, BTU/h, tons of refrigeration), and energy (kWh, BTU, joules, therms, kcal). The engineering unit converter hub covers the rest — dynamic and kinematic viscosity, length, area, and volume — behind a single quantity selector.
Two habits the tools encourage: convert once, through the base unit, rather than chaining approximate two-step conversions from memory; and keep measurement basis separate from unit conversion — the pressure tool converts psi to bar but deliberately does not convert gauge to absolute, just as the power tool distinguishes mechanical from metric horsepower instead of blurring the 1.4% gap. Each converter also publishes a contract-tested conversion chart for its highest-traffic pair, so the printed table can never drift from what the live tool computes.
All Unit Converters Tools
Pressure Unit Converter — psi, bar, kPa, MPa, atm & More
Pressure conversion between psi, bar, kPa, MPa, Pa, ksi, atm, Torr (mmHg), inches of water and kgf/cm² through the SI base unit (pascal) with the factors published in NIST SP 811 Appendix B — 1 psi = 6,894.757 Pa, 1 bar = 100,000 Pa exact, 1 atm = 101,325 Pa exact, 1 kgf/cm² = 98,066.5 Pa exact — showing the converted value plus an all-units table at 6 significant digits, accepting negative (vacuum-side gauge) readings, and publishing a psi-to-bar/kPa/atm conversion chart whose every cell is contract-tested through the tool's own conversion function.
Flow Rate Converter — CFM, GPM, L/s, m³/h & More
Volumetric flow conversion between CFM, US GPM, L/s, L/min, m³/s, m³/h and ft³/s through the SI base unit (cubic meter per second) with the factors published in NIST SP 811 Appendix B — 1 CFM = 4.719474×10⁻⁴ m³/s, 1 US gal/min = 6.309020×10⁻⁵ m³/s, the metric ladder exact by definition — showing the converted value plus an all-units table at 6 significant digits, rejecting negative magnitudes, keeping the US-vs-imperial gallon distinction honest, and publishing a CFM-to-m³/h/L/s/GPM conversion chart whose every cell is contract-tested through the tool's own conversion function.
Temperature Converter — Celsius, Fahrenheit, Kelvin & Rankine
Temperature conversion between °C, °F, K and °R through a Celsius pivot with the exact scale relations (°F = °C × 1.8 + 32; K = °C + 273.15; °R = °F + 459.67 — no rounded constant anywhere), rejecting inputs below absolute zero (−273.15 °C / 0 K / −459.67 °F / 0 °R) with a clear message, clamping the value to the new scale's floor when the from-scale switches so a valid entry never becomes an error, showing an all-scales table at 6 significant digits, explaining the temperature-difference trap (Δ°F = Δ°C × 1.8, no +32), and publishing a Fahrenheit-to-Celsius/Kelvin chart whose every cell is contract-tested through the tool's own conversion function.
Torque Unit Converter — Nm, ft-lb, in-lb & kgf·m
Torque conversion between N·m, kN·m, ft·lb, in·lb, kgf·m and kgf·cm through the SI unit (newton-meter) with the factors published in NIST SP 811 Appendix B — 1 ft·lbf = 1.355818 N·m, 1 in·lbf = 0.1129848 N·m (their 12:1 ratio exact), 1 kgf·m = 9.80665 N·m exact from standard gravity — showing the converted value plus an all-units table at 6 significant digits, accepting signed (negative) torque for direction conventions, calling out the 12× in-lb-vs-ft-lb stripped-thread trap, and publishing an Nm-to-ft-lb/in-lb/kgf·m conversion chart whose every cell is contract-tested through the tool's own conversion function.
Power Unit Converter — kW, HP, BTU/h & Tons of Refrigeration
Power conversion between W, kW, MW, mechanical horsepower (550 ft·lbf/s = 745.6999 W), metric horsepower (PS = 735.4988 W — kept honestly distinct, a 1.4% gap the tool never blurs), BTU/h, tons of refrigeration (12,000 BTU/h = 3,516.853 W) and kcal/h (1.163 W exact) through the SI base unit (watt) with the factors published in NIST SP 811 Appendix B, showing the converted value plus an all-units table at 6 significant digits, noting that rating bases (SAE vs DIN, input vs output) are a separate check from unit arithmetic, and publishing a kW-to-HP/PS/BTU-h conversion chart whose every cell is contract-tested through the tool's own conversion function.
Energy Unit Converter — kWh, BTU, Joules, Therms & kcal
Energy conversion between joules, kJ, MJ, Wh, kWh, BTU (IT), kcal (IT), ft·lbf and US therms through the SI base unit (joule) with the factors published in NIST SP 811 Appendix B — 1 kWh = 3.6×10⁶ J exact, 1 BTU (IT) = 1,055.056 J, 1 kcal (IT) = 4,186.8 J exact, 1 US therm = 1.054804×10⁸ J exact — showing the converted value plus an all-units table at 6 significant digits, keeping the gas-vs-electric anchors honest (1 kWh ≈ 3,412.14 BTU; 1 therm ≈ 29.3001 kWh), separating energy (kWh) from power (kW) explicitly, and publishing a kWh-to-BTU/MJ/kcal conversion chart whose every cell is contract-tested through the tool's own conversion function.
Engineering Unit Converter — Pressure, Flow, Viscosity, Length & More
Multi-quantity hub for the units without a dedicated converter: dynamic viscosity (Pa·s, cP, poise, lbf·s/ft² = 47.88026 Pa·s), kinematic viscosity (m²/s, cSt, stokes, ft²/s = 0.09290304 m²/s exact), length (mm to miles on the exact 1959 inch family), area (mm² to acres at NIST's US-survey 4,046.873 m² and hectares) and volume (in³, ft³, US gallons, liters, m³, yd³) — each converting through its SI base unit with NIST SP 811 Appendix B factors behind a quantity selector that resets the unit selects on switch so a stale key never leaks across sets, with the ν = μ/ρ dynamic-vs-kinematic bridge explained and an acres-to-ft²/m²/ha chart whose every cell is contract-tested through the tool's own conversion function.
Mass & Weight Converter — kg, lb, oz, Tons & Stone
Mass conversion between kilograms, grams, metric tonnes, pounds, ounces, stone, US short tons and UK long tons through the SI base unit (kilogram) with the exact avoirdupois definitions in NIST SP 811 Appendix B — 1 lb = 0.45359237 kg exact (1959 international pound), 1 oz = 1/16 lb, 1 stone = 14 lb = 6.35029318 kg (NIST HB 44 App. C), 1 short ton = 2,000 lb = 907.18474 kg, 1 long ton = 2,240 lb = 1,016.0469088 kg — showing the converted value plus an all-units table at 6 significant digits, keeping the three tons explicitly distinct so a bare "ton" never hides a 10% gap, and publishing a kg-to-lbs/oz/stone conversion chart whose every cell is contract-tested through the tool's own conversion function.
Force Unit Converter — N, lbf, kgf, kN & kip
Force conversion between newtons, kilonewtons, pounds-force, kips, kilograms-force and dynes through the SI unit (newton) with the factors published in NIST SP 811 Appendix B.9 — 1 lbf = 4.448222 N (exact 4.4482216152605), 1 kip = 1,000 lbf = 4,448.222 N, 1 kgf = 9.80665 N exact from standard gravity, 1 dyne = 10⁻⁵ N exact — showing the converted value plus an all-units table at 6 significant digits, accepting signed (negative) forces for tension/compression conventions, keeping the mass-vs-force distinction explicit (kgf is a force, kg is not), and publishing an N-to-lbf/kgf conversion chart whose every cell is contract-tested through the tool's own conversion function.
Speed Converter — mph, km/h, m/s, Knots & ft/s
Speed conversion between miles per hour, kilometers per hour, meters per second, feet per second, feet per minute and knots through the SI unit (meter per second) with the factors published in NIST SP 811 Appendix B.9 — 1 mph = 0.44704 m/s exact via the 1959 international foot, 1 ft/s = 0.3048 m/s exact, 1 km/h = 1/3.6 m/s exact, 1 knot = 1,852/3,600 m/s exact from the 1,852 m nautical mile — showing the converted value plus an all-units table at 6 significant digits, keeping speed a magnitude (negative values rejected; direction belongs to the vector), and publishing an mph-to-km/h/m-s/knots conversion chart whose every cell is contract-tested through the tool's own conversion function.
Density Converter — kg/m³, g/cm³, lb/ft³ & lb/gal
Density conversion between kg/m³, g/cm³, g/mL, lb/ft³, lb/US gal, lb/in³ and slug/ft³ through the SI unit (kilogram per cubic meter) with the factors published in NIST SP 811 Appendix B.9 — 1 lb/ft³ = 16.01846 kg/m³, 1 lb/US gal = 119.8264 kg/m³ (231 in³ gallon), 1 lb/in³ = 27,679.90 kg/m³, 1 slug/ft³ = 515.3788 kg/m³, g/cm³ = g/mL = 1,000 kg/m³ exact — showing the converted value plus an all-units table at 6 significant digits, keeping the US-vs-imperial gallon and the g/cm³-equals-specific-gravity anchors explicit, and publishing a water/steel/aluminum/concrete material-density chart whose every cell is contract-tested through the tool's own conversion function.
Angle Converter — Degrees, Radians, Grads & Slope Percent
Angle conversion between degrees, radians, grads (gons), arcminutes, arcseconds and revolutions through the degree with NIST SP 811 Appendix B.8 factors — 1 grad = 0.9° exact, 1° = 60′ = 3,600″ exact, 1 revolution = 360° = 2π rad exact, the radian on the exact 180/π ≈ 57.29578° identity — plus a deliberately separate slope panel for the NONLINEAR angle ↔ slope-percent ↔ rise:run relationship (slope % = tan θ × 100, so 45° = 100% = 1:1 and 8.333% = 4.7636° = the ADA-familiar 1:12), guarded at ±89.99° where the tangent diverges with the slope input capped to the matching percent, and publishing a degrees-to-slope-percent/radians chart whose every cell is contract-tested through the tool's own conversion functions.
Frequency & Rotation Converter — Hz, RPM & rad/s
Frequency and rotational-speed conversion between hertz, kilohertz, megahertz, RPM, radians per second and revolutions per day through the SI unit (hertz) with exact identities — 1 RPM = 1/60 Hz, 1 rad/s = 1/(2π) Hz via the 2π rad per revolution of NIST SP 811 Appendix B.8 (r/min → rad/s = 0.1047198), 1 rev/day = 1/86,400 Hz — showing the converted value plus an all-units table at 6 significant digits, keeping the cycles-vs-radians 2π distinction explicit (60 Hz = 376.991 rad/s), anchoring the 50/60 Hz motor synchronous-speed ladder (3,600/3,000/1,800/1,500 RPM), and publishing an RPM-to-Hz/rad-s conversion chart whose every cell is contract-tested through the tool's own conversion function.
Frequently Asked Questions
Why do the converters insist on going through an SI base unit?
Because pairwise factor tables rot. With ten pressure units there are 90 direct pairs to keep consistent, but only 10 factors into the pascal — and NIST SP 811 publishes exactly those. Converting value × f(from) ÷ f(to) through the base unit guarantees every pair is consistent with every other and that round-trips return your original number to machine precision.
Which conversion factors are exact, and which are rounded?
Definitions are exact: 1 bar = 10⁵ Pa, 1 atm = 101,325 Pa, 1 kWh = 3.6 MJ, 1 kgf = 9.80665 N, 1 in = 25.4 mm, 1 kcal (IT) = 4,186.8 J. Units defined in the other system are published rounded to 7 significant figures: 1 psi = 6,894.757 Pa, 1 BTU (IT) = 1,055.056 J, 1 hp = 745.6999 W. The tools use the NIST SP 811 Appendix B values either way and say which is which.
Why does temperature get its own converter instead of the generic one?
Temperature scales are affine, not linear: °F = °C × 1.8 + 32 has an offset, so you cannot express Fahrenheit as a single multiply-by factor. The temperature tool converts through a Celsius pivot with the exact offsets and additionally rejects inputs below absolute zero (−273.15 °C / 0 K / −459.67 °F), which a factor table could not do.