Measurement guide

Common Unit Conversions

Q: Is a pound of mass the same as a pound-force?

No. An avoirdupois pound is a mass unit, with 1 lb=0.45359237 kg. Pound-force is a force unit defined using standard gravity. A mass converter should not silently treat the two as interchangeable. Likewise, an ounce of mass cannot become a fluid ounce of volume without knowing the material density and the intended fluid-ounce convention.

Check factors, offsets and references.

Open measurement converters See conversion chart

Common unit conversion chart

Convert mass, temperature, speed and pressure by choosing the right relationship first. Most unit changes use a factor. Temperature readings need an offset, and pressure readings need a clearly stated reference.

Use the chart for quick lookups, then follow the worked examples to see unit cancellation, reverse checks and sensible rounding. Each example link opens a free converter with the inputs already selected. Examples use nominal values to make the arithmetic reproducible; real measurements retain their original precision.

Keep the full source unit name with the value. This chart uses international length definitions and avoirdupois mass unless stated otherwise. The pressure factors change units only; gauge and absolute reference choices are handled separately below.

Exact relationships and explicitly rounded approximations
Quantity or unit	Relationship	Convention
Avoirdupois pound	$1\,\mathrm{lb}=0.45359237\,\mathrm{kg}$	Exact; ordinary mass, not pound-force.
Avoirdupois ounce	$1\,\mathrm{oz}=28.349523125\,\mathrm g$	Exact; not a troy ounce or a fluid ounce.
Metric tonne	$1\,\mathrm t=1000\,\mathrm{kg}$	Exact; label tonne explicitly.
US short ton	$1\,\mathrm{ton}_{\mathrm{US}}=907.18474\,\mathrm{kg}$	Exact; two thousand avoirdupois pounds.
Imperial long ton	$1\,\mathrm{ton}_{\mathrm{Imp}}=1016.0469088\,\mathrm{kg}$	Exact; two thousand two hundred forty pounds.
Statute mile per hour	$1\,\mathrm{mph}=1.609344\,\mathrm{km/h}$	Exact; international mile.
Road speed to SI	$1\,\mathrm{km/h}=\frac{5}{18}\,\mathrm{m/s}$	Exact; divide the numerical value by 3.6.
Nautical knot	$1\,\mathrm{kn}=1.852\,\mathrm{km/h}$	Exact; one international nautical mile per hour.
Standard atmosphere	$1\,\mathrm{atm}=101.325\,\mathrm{kPa}$	Exact defined unit; not every local atmospheric pressure.
Bar	$1\,\mathrm{bar}=100\,\mathrm{kPa}$	Exact; bar and standard atmosphere differ.
Hectopascal and millibar	$1\,\mathrm{hPa}=1\,\mathrm{mbar}=100\,\mathrm{Pa}$	Exact; commonly used for weather.
Pound-force per square inch	$1\,\mathrm{psi}\approx6.894757293\,\mathrm{kPa}$	Rounded decimal; defined using standard gravity and the international inch.

Factor references: NIST conversion method and NIST quantity tables. Length, area and liquid-volume charts appear in our Metric vs Imperial guide.

Unit relationship diagram for pounds to kilograms, Celsius to Fahrenheit, miles per hour to kilometers per hour, and the difference between gauge and absolute pressure. — Representative exact factors and formulas from the mass, temperature, speed, and pressure guide.

Choose the conversion method before calculating

Identify the quantity and convention. A pound of mass, a pound-force and a fluid ounce describe different quantities. Check whether a temperature is a reading or a change, and whether pressure is gauge, absolute or a difference.
Arrange units to cancel. Put the unwanted unit in the denominator of a conversion factor. For speed, convert both distance and time. If the surviving unit is not the intended answer unit, fix the setup before multiplying.
Apply offsets only when needed. Temperature readings use scale-specific zero points. A pressure-reference conversion uses the stated local atmosphere. These additions are not ordinary multiplicative unit changes.
Check magnitude, reverse and round. Convert back with unrounded values and compare with the original input. Keep guard digits until the final step, then report units and measurement precision together.

150\,\cancel{\mathrm{lb}}\times\frac{0.45359237\,\mathrm{kg}}{1\,\cancel{\mathrm{lb}}}=68.0388555\,\mathrm{kg}

The cancellation marks show why this factor is multiplied. Flipping the factor would leave the wrong units. An exact definition supports an exact nominal calculation, but it does not imply a weighing instrument measured all the digits shown.

Example: pounds to kilograms

A shipping record lists a nominal mass of $150\,\mathrm{lb}$ . Find kilograms using the international avoirdupois pound, the convention used for ordinary shipping and body-mass labels.

Select the exact definition: $1\,\mathrm{lb}=0.45359237\,\mathrm{kg}$ .
Multiply the numerical mass: $150\times0.45359237=68.0388555$ . Attach kilograms to the result.
For a display rounded to two decimal places, report $68.04\,\mathrm{kg}$ . Check the unrounded answer by dividing: $68.0388555/0.45359237=150$ .

Do not use a rounded mental estimate as the stored conversion factor. For precious metals, choose troy units explicitly. For a label that just says “ton,” establish whether it means US short ton, Imperial long ton or metric tonne before converting.

Check this pounds-to-kilograms example

Example: a temperature reading versus a change

Let $c$ and $f$ be the numerical readings in degrees Celsius and Fahrenheit. The formulas apply to those scale readings; they are not dimensionless temperature quantities. Kelvin readings use $T$ in the final relationship below.

f=\frac95c+32\qquad c=\frac59(f-32)

T=(c+273.15)\,\mathrm K

Start with a reading of $77\,{}^\circ\mathrm F$ . Subtract the Fahrenheit offset: $77-32=45$ .
Apply the scale factor: $45\times5/9=25$ . The equivalent reading is $25\,{}^\circ\mathrm C$ .
Check the reverse: $25\times9/5+32=77$ . The same temperature is $298.15\,\mathrm K$ .

Now consider a rise from $20\,{}^\circ\mathrm C$ to $30\,{}^\circ\mathrm C$ . The change is $10\,{}^\circ\mathrm C$ , so the Fahrenheit change is $18\,{}^\circ\mathrm F$ . The readings are $68\,{}^\circ\mathrm F$ and $86\,{}^\circ\mathrm F$ ; subtracting them cancels the offset.

\Delta f=\frac95\Delta c=\frac95(10)=18

A negative temperature change is a fall and remains valid. A kelvin temperature reading below absolute zero is outside this ordinary conversion model. Kelvin uses no degree symbol. NIST explains the scale relationships.

Check the temperature reading Check the temperature change

Example: miles per hour to SI speed

Convert a nominal speed of $60\,\mathrm{mph}$ to both kilometers per hour and meters per second. Use the international statute mile, not the nautical mile. An hour contains exactly $3600$ seconds.

For road-speed units, use $1\,\mathrm{mi}=1.609344\,\mathrm{km}$ . $60\,\mathrm{mph}\times\frac{1.609344\,\mathrm{km}}{1\,\mathrm{mi}}=96.56064\,\mathrm{km/h}$
For meters per second, convert the distance and the time denominator. $\frac{60\,\mathrm{mi}}{1\,\mathrm h}\times\frac{1609.344\,\mathrm m}{1\,\mathrm{mi}}\times\frac{1\,\mathrm h}{3600\,\mathrm s}=26.8224\,\mathrm{m/s}$
Check independently: $96.56064/3.6=26.8224$ . Rounded to two decimal places, the SI result is $26.82\,\mathrm{m/s}$ .

For motion calculations, convert speed before using it with time in seconds or acceleration in SI units. A knot uses a nautical mile, and Mach needs the local speed of sound; neither can be substituted for statute miles per hour without changing the definition.

Check speed in meters per second Check road-speed units

Example: psi to kPa, then gauge to absolute

Suppose a gauge reads $35\,\mathrm{psi}$ . First convert units without changing its reference. The exact psi factor follows from pound-force at standard gravity and the international inch; the decimal shown here is rounded.

p_{\mathrm g}=35\,\mathrm{psi}\times\frac{6.894757293\ldots\,\mathrm{kPa}}{1\,\mathrm{psi}}\approx241.317\,\mathrm{kPa}

That answer is still gauge pressure. To obtain absolute pressure, assume a local atmospheric pressure of $100\,\mathrm{kPa}$ for this example. Add it to the unrounded gauge result, not to the original value in psi.

p_{\mathrm{abs}}=p_{\mathrm g}+p_{\mathrm{atm}}

p_{\mathrm{abs}}\approx241.317\,\mathrm{kPa}+100\,\mathrm{kPa}=341.317\,\mathrm{kPa}

The gold bar begins at the assumed local atmosphere; the black bar begins at vacuum. Adding the atmospheric part changes the reference. Changing units alone leaves that reference unchanged. This is a linear pressure comparison, not a diagram of a physical gauge.

The assumed local atmosphere is intentionally different from the defined standard atmosphere, $1\,\mathrm{atm}=101.325\,\mathrm{kPa}$ . Local atmospheric pressure changes with altitude and weather. Use the measured reference when a real calculation depends on it. Subtracting that same reference from the absolute result recovers the gauge pressure.

Negative gauge pressure can describe a partial vacuum; zero gauge pressure means equality with the local atmosphere, not zero absolute pressure. The atmospheric term is absent when merely converting a pressure difference. OpenStax explains the reference distinction.

Convert psi to kPa Check gauge-to-absolute pressure Convert one standard atmosphere

Report precision and unit labels honestly

A calculator can show more digits than the original measurement justifies. Treat exact definitions differently from measured inputs: carry extra digits during arithmetic, then round the final value according to the measurement uncertainty or the reporting requirements. A short rounded display is useful, but do not feed it into a reverse calculation to judge the accuracy of the underlying factor.

Prefixes are case-sensitive. Milli and mega do not mean the same thing. Area and volume factors also need the corresponding power of the length factor; applying a length conversion once to a square or cubic measure gives the wrong dimensions. See the worked dimensional examples in Metric vs Imperial and the precision guidance in Lab Report Calculations.

For recipes, label US liquid or Imperial capacity and keep mass separate from volume. Ingredient density is needed to convert between grams and a volume measure. Our Kitchen Conversions guide explains that additional assumption. For measured results, use the Significant Figures Calculator to check presentation after completing the conversion.

Practice with worked answers

Try these nominal-value exercises before opening the answers. State the conversion direction and the pressure reference, then check the remaining units.

Convert two avoirdupois pounds to grams.

2\,\mathrm{lb}\times\frac{0.45359237\,\mathrm{kg}}{1\,\mathrm{lb}}\times\frac{1000\,\mathrm g}{1\,\mathrm{kg}}=907.18474\,\mathrm g

Use the exact kilogram definition first, then the gram prefix. This is mass; no density assumption is needed.

Convert ninety kilometers per hour to meters per second.

90\,\mathrm{km/h}\times\frac{1000\,\mathrm m}{1\,\mathrm{km}}\times\frac{1\,\mathrm h}{3600\,\mathrm s}=25\,\mathrm{m/s}

Hours cancel from the denominator. Multiplying by seconds per hour instead would produce the wrong dimensions.

Find gauge pressure from 250 kilopascals absolute with a local atmosphere of 100 kilopascals.

p_{\mathrm g}=250\,\mathrm{kPa}-100\,\mathrm{kPa}=150\,\mathrm{kPa}

Subtract the local atmospheric reference because the input is absolute. Label the answer as gauge pressure; the unit alone does not carry that information.

Common conversion questions

Should I multiply or divide to convert units?

Write the conversion factor as a fraction with the original unit in its denominator and the desired unit in its numerator. The original units then cancel. For pounds to kilograms, multiply by the kilograms-per-pound factor. To reverse that conversion, divide by the same factor. Estimate the size first: the number of kilograms should be smaller than the number of pounds for the same positive mass.

Is a pound of mass the same as a pound-force?

No. An avoirdupois pound is a mass unit, with $1\,\mathrm{lb}=0.45359237\,\mathrm{kg}$ . Pound-force is a force unit defined using standard gravity. A mass converter should not silently treat the two as interchangeable. Likewise, an ounce of mass cannot become a fluid ounce of volume without knowing the material density and the intended fluid-ounce convention.

Does a temperature change use the Fahrenheit offset?

No. The offset applies when converting a temperature reading between scales with different zero points. When you subtract two readings, that offset cancels. A rise of $10\,{}^\circ\mathrm C$ is a rise of $18\,{}^\circ\mathrm F$ . Use the difference mode in the temperature converter and retain the word rise, fall or change so the result cannot be mistaken for a final reading.

Does converting psi to kPa change gauge pressure into absolute pressure?

No. A unit change preserves the pressure reference. If the input is gauge pressure, its value in kilopascals remains gauge pressure. To calculate absolute pressure, also add the local atmospheric pressure expressed in matching units. State that reference explicitly; a standard atmosphere is a defined unit and does not automatically describe local weather or altitude.

Are all conversion factors exact?

Many unit relationships are defined exactly, such as the international pound, inch and mile. A displayed decimal may still be rounded, especially when the exact relationship is a repeating fraction. Other conventions, including some mercury-column factors, depend on an adopted definition or table. Exact factors never create extra measurement precision: keep guard digits during arithmetic and round the final measured result appropriately.

Can I use one fixed factor for Mach or for gallons?

Not without context. Mach is a ratio to the local speed of sound, which depends on the medium and conditions, so it needs an explicit sound-speed reference. Gallons must be labeled as US liquid, US dry or Imperial. These ambiguities concern the meaning of the source value; a calculator cannot recover a missing convention from the number alone.

Sources and calculation conventions

Reviewed against the references below. Worked examples are original nominal calculations, with exact factors retained internally. Temperature formulas refer to numerical scale readings. Pressure examples identify whether the operation changes units, reference, or both.

NIST: conversion-factor method
Multiplication, reciprocal factors, derived units and rounding guidance.
NIST: factors by physical quantity
Mass, speed and pressure tables. Some printed decimals are rounded; exact definitions are retained in our calculations.
NIST: temperature scales
Celsius, Fahrenheit and kelvin relationships, including scale offsets.
BIPM: SI Brochure
SI units, prefixes and the distinction between base and derived units.
OpenStax: measuring pressure
Gauge pressure, atmospheric reference and absolute pressure.

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Written and maintained by the Toolarithm editorial team. The method was last reviewed on October 30, 2026. No independent professional review is claimed.

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Published June 10, 2026. Updated October 30, 2026.