Ounces to Grams Calculator

Background.

An ounces-to-grams calculator earns its keep at the shipping counter. US postal and courier services weigh small parcels in ounces and price them against weight-break tiers (up to 1 oz, up to 4 oz, up to 16 oz, and so on), but international customs forms, metric-country carriers, and many e-commerce platforms expect that same weight in grams. A 16 oz package — exactly 1 pound — needs to be declared as 453.59237 g on a form that only accepts metric units, and getting that figure exactly right avoids a customs delay or a rate miscalculation on the receiving end.

The factor behind the conversion, 1 ounce = 28.349523125 grams, is not a rounded shortcut. It falls directly out of the exact 1959 international definition of the avoirdupois pound (0.45359237 kg, divided into 16 ounces), so the conversion carries no measurement uncertainty — only the arithmetic, which this calculator performs in arbitrary-precision decimal math to avoid the small drift that ordinary floating-point division can introduce, especially when a shipment's weight is built up from many small item weights added together.

The same factor applies whether you are declaring a single small parcel or converting a bulk shipment's weight for a metric-denominated freight quote, so this calculator scales cleanly from a single letter to a multi-item package.

What is ounces to grams calculator?

Ounces and grams both measure mass, but they belong to different systems: the avoirdupois ounce is a US customary unit equal to 1/16 of a pound, while the gram is a decimal subdivision of the SI kilogram. The two are linked through the same constant that defines the pound: because 1 avoirdupois pound is exactly 0.45359237 kilograms (fixed by the 1959 international yard-and-pound agreement) and a pound contains exactly 16 ounces, 1 ounce is exactly 0.45359237 ÷ 16 = 28.349523125 grams. That figure is a mathematical consequence of a legal definition, not a rounded approximation, so it is exact to as many decimal places as needed. This calculator converts ounces to grams by multiplying the ounce value by 28.349523125, producing a precise gram figure suitable for customs declarations, metric shipping-rate tables, or postal weight-break comparisons.

How to use this calculator.

1. 01Enter the weight in ounces — the default (16 oz) is exactly 1 pound, a common postal weight-break point.
2. 02Read the result: the equivalent weight in grams, computed to full decimal precision.
3. 03Round to the precision your shipping form or rate table requires (most accept whole grams or one decimal place).
4. 04Repeat for each parcel or item to build a metric-weight shipping manifest from ounce-based scale readings.

The formula.

The calculator uses one fixed constant: 1 avoirdupois ounce equals exactly 28.349523125 grams. That number is derived, not independently defined — it comes directly from the 1959 international agreement that fixed the avoirdupois pound at exactly 0.45359237 kilograms, divided by the standard 16 ounces per pound: 0.45359237 kg ÷ 16 = 28.349523125 g. Because both inputs to that derivation are exact legal definitions, the resulting factor is exact too.

To convert ounces to grams, the calculator multiplies the ounce value by 28.349523125: grams = ounces × 28.349523125. For the 16 oz worked example, that gives exactly 453.59237 g — note this equals the exact kilogram-to-gram value of one avoirdupois pound (0.45359237 kg × 1000), which is the built-in consistency check for the whole mass-conversion family. The calculation runs through Decimal.js so the result holds full precision rather than the rounding noise ordinary floating-point multiplication can introduce at scale.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Bureau of Standards (1959). Refinement of Values for the Yard and the Pound — establishes the international avoirdupois pound as exactly 0.45359237 kilograms, from which the ounce (1/16 lb = 28.349523125 g) is derived.
2. [2]National Institute of Standards and Technology (2008). NIST Special Publication 811: Guide for the Use of the International System of Units (SI), Appendix B — Conversion Factors.