How our conversions are derived
Every number CupUnit publishes can be traced to a named source, one documented convention and exact arithmetic. This page explains the whole pipeline — the same rules the converter applies on every page.
Where the values come from
Each ingredient’s reference weight is reconciled from published primary sources: the USDA FoodData Central household measures, the King Arthur Baking ingredient weight chart, and NIST definitions for cup volumes and unit conversions. We never average sources silently — we pick one published figure, name it, and list the alternatives openly.
One reference convention
Every ingredient stores one reference weight: grams per one US cup of 240 ml, the cup USDA household measures use, together with the measuring method that weight assumes (spooned and levelled, packed, scooped, or liquid measure). Stating the method matters more than the number itself — a cup of flour can vary by 30 g depending on how it is filled.
Scaling to other cup standards
Other cups are scaled exactly from their legal volumes: US legal cup 240 ml, US customary cup 236.588 ml, metric cup 250 ml, imperial cup 284.131 ml. We derive a full-precision density (reference weight ÷ 240 ml) and multiply by the exact cup volume — rounded display values are never fed back into calculations.
Tablespoons and teaspoons
Spoon conversions use the exact US culinary definitions: one tablespoon is 14.78676 ml (16 to a cup) and one teaspoon is 4.92892 ml (3 to a tablespoon). The same density logic applies.
Rounding
Displayed results are rounded to practical kitchen precision (whole grams by default), because implying tenth-of-a-gram accuracy for a volume measure would be false precision. The converter also shows a more precise value for scaling recipes.
Why we show alternative values
Reputable sources legitimately disagree — a cup of flour is 120 g at King Arthur and 125 g at USDA because they assume different filling methods. Wherever that happens, we publish the value we chose, say why, and list the alternatives with their sources instead of pretending there is one true number.
Review and versioning
The dataset is versioned (currently v2.2.0) and each value was last reconciled against its sources on 2026-08-15. Automated tests enforce that the visible pages, the machine-readable dataset and this documentation never disagree.
Limits of any cup conversion
A cup measures volume, not weight, so every conversion here is a close estimate, not an exact truth. Brand, humidity, grind and packing all shift real-world weights by a few percent. For critical baking, weigh ingredients directly.
Machine-readable dataset
The full dataset — densities, all cup standards, sources and version — is public JSON, free to cite: