Daltons appear beside molecular masses, in protein descriptions and in analytical discussions. Milligrams appear beside the amount of material supplied. Both concern mass, but they describe different scales and objects. Keeping that distinction clear prevents a molecular-size field from being mistaken for a vial-content result.
What a dalton represents
BIPM recognises the dalton, symbol Da, as a mass unit. It is equal to one twelfth of the mass of a free carbon-12 atom at rest in its ground state; the unified atomic mass unit, u, is an alternative name and symbol for the same unit.BIPM — The International System of Units (opens in a new tab)
This provides a convenient scale for describing very small entities. A molecular mass in daltons is not the weight of a bottle, a package or a whole powder sample. Read which molecule or other entity the value refers to before using it.
A paper may discuss the mass of a single chain, a modified form or a larger assembly. The unit remains a mass unit, but the object being described changes. The accompanying identity is therefore as important as the number.
Convert the prefix without changing the entity
The SI prefix kilo represents a factor of 1,000. Thus 1 kDa = 1,000 Da. This conversion changes the numerical scale, not the molecular description.BIPM — SI prefixes (opens in a new tab)
| Mass in Da | Same mass in kDa |
|---|---|
| 750 Da | 0.750 kDa |
| 2,400 Da | 2.400 kDa |
| 18,500 Da | 18.500 kDa |
The displayed digits in this teaching table make the scale change easy to see; they are not claims about measurement precision. When converting a real result, retain the precision justified by its source rather than adding trailing zeroes because a calculator allows it.
As a quick directional check, the number should become smaller when moving from daltons to kilodaltons. If 2,400 Da becomes 2,400,000 kDa, the prefix operation has gone the wrong way.
Molecular mass is not sample content
An illustrative peptide might be described by a molecular mass of 2.4 kDa while a supplied item is labelled with an amount in milligrams. Those fields answer different questions. One describes the molecular entity; the other is an amount claim for the supplied material.
A molecular-mass field cannot confirm that the labelled amount is present. Conversely, a content measurement in milligrams does not, by itself, establish the sequence or exact molecular form. Keep both fields rather than choosing one as a substitute for the other.
Read the analytical heading around the value
A figure may use a molecular-size scale, a calculated mass annotation or an observed analytical result. These are not interchangeable sources of evidence. Identify whether the number was calculated from a proposed structure or derived through a stated measurement.
Mass-spectrometry plots also require care because an ion's mass-to-charge ratio is a different quantity from neutral molecular mass. Do not copy an axis number into a Da field without reading how the authors interpreted the ions. The MS guide covers that distinction in detail.
If two sources give different values, check their molecular forms and mass conventions before assuming a disagreement in identity. Differences in termini or other declared features may be relevant, but they must be documented rather than invented to reconcile the numbers.
Write an unambiguous mass entry
Record the entity, numerical value, unit and provenance together. A concise entry such as “calculated mass for the stated chain, value in Da” is more useful than an unexplained number. Add the source and any unresolved convention when the distinction matters to the research question.
Sources and further detail
- BIPM — The International System of Units (opens in a new tab)
Ninth edition, English version 4.01 (2026), tables 7 and 8 and section 5.4.7. Definitions and notation; examples in this article are independently constructed.
- BIPM — SI prefixes (opens in a new tab)
Official decimal factors and case-sensitive symbols.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.