Weight Conversion Explained

Why weight conversion is trickier than it looks

Converting between kilograms and pounds, or grams and ounces, is one of the most common everyday calculations on the internet. The math is multiplication, not magic. The complications come from the units themselves — there are at least three different "tons", two different "ounces", and a "stone" that almost no one outside the UK and Ireland recognizes. The weight converter on Calc Dragon handles every common avoirdupois and metric unit and a few compound formats, but it is worth understanding the conversion factors so the numbers make sense rather than appearing as black-box outputs.

This article walks through the underlying conversion factors, the difference between mass and weight (one of the few points where physics actually intrudes on daily life), the avoirdupois versus troy distinction that catches people buying gold, and the three kinds of ton in active use. It also covers why a kilogram is no longer defined by a chunk of metal in a Paris vault, what the 1959 International Yard and Pound Agreement fixed, and how to do common conversions in your head without reaching for a calculator.

The math behind every weight conversion

Every conversion in the weight converter uses a single intermediate unit: the gram. Each unit has a "grams per unit" factor, and the conversion is two multiplications:

result = value × (grams per source unit) ÷ (grams per target unit)

So 70 kg expressed in pounds is 70 × 1000 ÷ 453.59237 ≈ 154.324 lb. The same gram-bridge handles every pair without needing nine factors for nine target units — only one factor per unit is stored, and every other conversion follows from it. This is the standard approach in scientific software, units libraries (like Python's pint or JavaScript's convert-units), and the SI brochure itself.

The factors used are exact wherever possible. The avoirdupois pound has been defined as exactly 0.45359237 kilograms since the 1959 International Yard and Pound Agreement, signed by the United States, the United Kingdom, Australia, Canada, New Zealand, and South Africa. Every avoirdupois ounce is therefore exactly 1/16 lb, every short ton is exactly 2000 lb, and every long ton is exactly 2240 lb. The metric tonne is exactly 1000 kg by definition. Only when displaying the result does any rounding occur — the underlying arithmetic is full floating-point precision.

Worked example: a 75 kg adult in three different units

Take a person who weighs 75 kilograms. The weight converter gives:

• In pounds: 75 × 1000 ÷ 453.59237 ≈ 165.347 lb
• In stone and pounds: 75 kg → 165.347 lb → 11 st 11.35 lb (since 1 stone = 14 lb, 165.347 ÷ 14 = 11 with 11.35 lb remainder)
• In ounces: 75 × 1000 ÷ 28.349523125 ≈ 2645.547 oz

The compound format ("11 st 11.35 lb") is the answer most people from the UK or Ireland want when reading a weight in stone — a flat decimal like "11.811 stone" is mathematically correct but unusable in conversation. The converter handles the decomposition exactly, rounding only the smallest unit to two decimal places to avoid awkward strings like "11 st 11.347 lb".

Going the other direction is symmetric. Converting 11 st 12 lb back to kilograms means first turning the compound value into a single unit (11 × 14 + 12 = 166 lb), then applying the gram-bridge (166 × 453.59237 ÷ 1000 ≈ 75.30 kg).

Mass versus weight: a distinction that matters in physics

Strictly speaking, "weight" is a force and "mass" is an amount of matter. A kilogram is a unit of mass. A newton is the SI unit of force, and a pound (in physics, "pound-force") is also a unit of force. In everyday English, "weight" and "mass" are used interchangeably, and bathroom scales report mass in kilograms or pounds even though they actually measure force.

The reason this works on Earth is that gravitational acceleration is roughly constant at the surface — about 9.81 m/s². A kilogram of mass weighs 9.81 newtons of force everywhere from sea level to the top of Mount Everest, give or take a fraction of a percent. Off Earth the distinction matters: a 75 kg astronaut on the Moon still has 75 kg of mass but weighs only about 12.4 kg-equivalent on a lunar bathroom scale, because lunar gravity is one sixth of Earth's.

For the kinds of conversions this calculator handles — body weight, parcel weight, recipe ingredients, shipping freight — the mass-versus-weight distinction is irrelevant in practice. The units and conversion factors used are mass units throughout, and the answer is the same on any planet you choose to do the arithmetic.

Avoirdupois versus troy: the precious-metals exception

There are two ounces in active commercial use. The avoirdupois ounce (abbreviated oz) is the one used for groceries, body weight, parcels, and almost everything else. It weighs exactly 28.349523125 g — 1/16 of an avoirdupois pound. This is the ounce the converter uses throughout.

The troy ounce (abbreviated oz t or just "ounce" in bullion markets) weighs 31.1034768 g — about 10% heavier. Troy ounces are used exclusively for precious metals: gold, silver, platinum, palladium. When a news report says "gold hit $2,400 an ounce," it always means a troy ounce. A troy pound, confusingly, is only 12 troy ounces (not 16), so a troy pound (373.24 g) is actually lighter than an avoirdupois pound (453.59 g) despite the heavier ounce.

The Calc Dragon weight converter uses avoirdupois throughout because that is what 99%+ of weight questions need. If you are converting bullion, multiply the troy figure by 31.1034768 to get grams, or divide by 0.911458 to convert troy ounces to avoirdupois ounces.

The three kinds of ton

The word "ton" is one of the most ambiguous in English measurement and the source of repeated confusion in shipping, freight, and engineering. There are three units in active use:

• Short ton (US ton): 2000 lb ≈ 907.185 kg. The default in the United States. When American shipping quotes "tons", this is almost always what they mean.
• Long ton (UK ton, imperial ton): 2240 lb ≈ 1016.047 kg. The traditional British ton, still used in some UK industries (notably shipping) and for some commodities like steel.
• Metric tonne (or tonne): exactly 1000 kg ≈ 2204.62 lb. The international standard, spelled "tonne" in British English to distinguish it from "ton" but often spelled "metric ton" in American English. This is what the SI uses and what every science and most commerce defaults to.

The gap between the three is large enough to matter. A 100,000-ton order of grain is 90,718,474 kg as a US short-ton order, but 100,000,000 kg as a metric tonne order — a difference of more than 9 million kg, or about 10%. International contracts almost always specify "metric tonnes" explicitly to remove the ambiguity. The weight converter exposes all three so you can check which you actually have.

Where the kilogram came from (and where it is now)

For most of its history, the kilogram was defined by a physical object: the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder kept in a vault outside Paris and only unlocked once every few decades. This was a problem. Comparing replicas to the IPK over a century showed it was drifting in mass, by a few tens of micrograms, and "the kilogram" was whatever the cylinder weighed on the day you measured it.

On 20 May 2019, World Metrology Day, the SI redefined the kilogram in terms of a physical constant — Planck's constant, fixed at exactly 6.62607015 × 10⁻³⁴ J·s. The kilogram is now defined indirectly through that constant and the existing definitions of the metre and the second. Any sufficiently accurate Kibble balance can realize a kilogram from first principles, with no reference to the cylinder in Paris. The practical effect on a bathroom scale is zero — the change was designed to be undetectable in everyday measurement — but it means weight conversion factors will not drift in the future the way the old IPK did.

How to convert weights in your head

Converting between kg and lb is one of the most common approximations to learn. The exact factor is 2.2046226 lb per kg, but for mental math the rule "double it and add 10%" is accurate to within a quarter of a percent:

• kg → lb: double the kg figure, then add 10%. So 70 kg → 140 + 14 = 154 lb. The exact answer is 154.32, so the mental shortcut is off by about 0.2%.
• lb → kg: divide by 2 and subtract 10%. So 200 lb → 100 − 10 = 90 kg. The exact answer is 90.72, so the rule underestimates by about 0.8%.
• kg → stone: divide by 6.35. For a quick estimate, divide kg by 6 and subtract a few percent. 70 kg → 70 ÷ 6 ≈ 11.67 stone, exact answer 11.02 stone. The rougher rule ("divide by 6.5") is closer for typical adult body weights.
• g → oz: divide by 28. 500 g → about 17.9 oz (exact: 17.64). For cooking, dividing by 30 is close enough and easier to do in your head.

The point of these mental rules is not to replace the converter — the exact answer is one input away — but to spot when a converted figure is obviously wrong. If a bag of flour is labelled "500 g / 5 oz", you know without checking that the conversion is off by a factor of three.

Common mistakes

Confusing oz with fl oz

An ounce (oz) is a unit of weight; a fluid ounce (fl oz) is a unit of volume. They are different units measuring different physical quantities, and they are not interchangeable. One US fluid ounce of water happens to weigh about one avoirdupois ounce, which is where the confusion comes from, but for any other liquid (or for UK fluid ounces, which are different again) the conversion breaks down. Volume conversions need a separate calculator entirely — see the distance converter for length and check back for volume tools.

Treating "ton" without specifying which

The 10% gap between US short tons and metric tonnes, and a further 1.6% gap to UK long tons, means a contract that says "ton" without specifying is a contract waiting for a dispute. Shipping bills, freight quotes, and commodity pricing should always say "tonne", "short ton", or "long ton" — never just "ton". The converter exposes all three so the answer is unambiguous.

Converting troy ounces with avoirdupois factors

A 1 oz (avoirdupois) gold coin would weigh 28.35 g. A 1 troy oz gold coin actually weighs 31.10 g. Plugging troy ounces into the weight converter as plain "oz" understates the weight by about 10%. Bullion always uses troy; almost everything else uses avoirdupois. Check the abbreviation ("oz t", "ozt", or "t oz" for troy) before converting.

Stripping units before doing the math

The classic engineering mistake is to do arithmetic on the numbers without tracking the units. NASA lost the Mars Climate Orbiter in 1999 because thrust data was transmitted in pound-force-seconds while the navigation software expected newton-seconds — a factor-of-4.45 mismatch that put the spacecraft into the Martian atmosphere instead of orbit. For spreadsheet work, label every column with units and make conversions explicit; for software, use a units library where possible.

When the converter is not enough

For physics or engineering work where mass and weight need to be treated separately, this converter is not the right tool — it treats every unit as mass and assumes Earth-surface gravity. For force, use a units library that handles newtons, pound-force, and kilogram-force as distinct dimensions. For very high-precision scientific work (sub-microgram), look up the latest CODATA constants rather than relying on a web converter; the values used here are correct to nine significant figures, which covers every commercial and consumer application but stops short of the precision a metrology lab would want.

For the day-to-day questions — "how many pounds is 70 kg", "what's 11 stone in kilos", "how many ounces in 500 grams" — the Calc Dragon weight converter gives the exact answer using NIST-published factors and the 1959 International Yard and Pound Agreement. The math is simple, the constants are exact, and the result is the same number every accurate converter on the internet should return.

Frequently asked questions

See the FAQ on the weight converter page for direct answers on how many pounds are in a kilogram, how to convert kilograms to stone and pounds, the difference between short tons / long tons / metric tonnes, whether ounces are always 28.35 grams, and how accurate the results are. The combined calculator and FAQ cover both quick-reference and deeper questions on unit conversion.