Combustion Analysis Calculator

Background.

Combustion analysis burns a weighed sample in excess oxygen, traps the carbon dioxide and water it produces, and works the sample's empirical formula back out of those two masses. Enter the sample mass and the two trap masses and this calculator returns the grams of carbon, hydrogen and oxygen in the sample, the corresponding percentages, and the empirical formula.

The carbon and hydrogen come out cleanly, because every carbon atom in the trapped CO₂ and every hydrogen atom in the trapped H₂O must have come from the sample. Multiply the CO₂ mass by 12.011/44.009 and the H₂O mass by 2.016/18.015 and you have them.

OXYGEN IS THE PART TO BE CAREFUL WITH, and it belongs here rather than in a footnote. Oxygen is never measured. The combustion stream is flooded with excess O₂, so the oxygen in the collected products cannot be attributed to the sample; sample oxygen is whatever mass the other elements fail to account for. ASTM's ultimate-analysis practice states the same thing for coal — there being no satisfactory direct test method for oxygen, it is calculated by subtracting everything else from 100. The consequence is that every error in the sample weight and in both trap weights accumulates in that one number, and a damp water trap can invent oxygen that was never there.

THE SCOPE LIMIT that follows: this page assumes the sample contains only carbon, hydrogen and possibly oxygen. If it contains nitrogen, sulfur, a halogen or leaves ash, those masses land in the 'oxygen' residual and the answer is wrong. Standardised ultimate analysis subtracts measured nitrogen, sulfur and ash before taking oxygen by difference; this calculator cannot, so use it only for C/H/O compounds. The hydrocarbon mode exists for the other common case: it fixes oxygen at zero and reports the leftover mass openly as a closure error, so you can see whether your no-oxygen assumption survives the numbers.

And remember what an empirical formula is. Burning butane, C₄H₁₀, returns C₂H₅ — the correct simplest ratio, and not the molecule. Fixing the molecular formula needs a separate molar-mass measurement, which the empirical formula calculator will combine with this result.

What is combustion analysis calculator?

Combustion analysis is the classical experiment for determining the elemental composition of an organic compound. A weighed sample is burned in a stream of excess oxygen; the products pass through an absorbent that traps water and then one that traps carbon dioxide, and each absorber is weighed before and after. Because complete combustion converts all the sample's carbon to CO₂ and all its hydrogen to H₂O, the two mass gains give the sample's carbon and hydrogen content directly through the gravimetric factors 12.011/44.009 and 2.016/18.015. Oxygen cannot be measured this way — the reaction runs in excess O₂ — so it is obtained by difference: sample mass minus carbon minus hydrogen. Converting those three masses to moles and reducing to the simplest whole-number ratio gives the empirical formula. The modern instrumental version of the same experiment is standardised as ASTM D5373, which determines carbon, hydrogen and nitrogen by trapping and quantifying CO₂, H₂O and N₂; oxygen is not among its determinands. For a molecular formula rather than a ratio, a molar mass from mass spectrometry is also required. The conventional bar for accepting a combustion analysis as evidence of purity is agreement with the calculated composition to within about 0.4 percentage points.

How to use this calculator.

1. 01Enter the mass of sample you burned, in grams. Weigh it accurately — this number sets the whole oxygen figure.
2. 02Enter the mass gained by the CO₂ absorber and the mass gained by the H₂O absorber, in grams.
3. 03Choose 'Carbon, hydrogen and oxygen' if the compound may contain oxygen, which is the usual case. The calculator finds the oxygen by difference.
4. 04Choose 'Hydrocarbon only' if you are asserting there is no oxygen. The leftover mass is then reported as an unexplained-mass figure instead, and if it is not close to zero, your assumption is wrong.
5. 05Read the empirical formula at the top and check the carbon and hydrogen percentages against any published analysis for the compound you suspect.
6. 06Take the empirical formula mass and a measured molar mass to the empirical formula calculator to get the molecular formula.
7. 07Do not use this page if the sample contains nitrogen, sulfur, a halogen or leaves ash — those masses would be mislabelled as oxygen.

The formula.

STEP 1 — carbon. Complete combustion sends every carbon atom in the sample to CO₂, so m(C) = m(CO₂) × A(C)/M(CO₂) = m(CO₂) × 12.011/44.009 = m(CO₂) × 0.272921. The factor is below 1 because CO₂ outweighs the carbon it carries.

STEP 2 — hydrogen. Every hydrogen atom ends up in H₂O, so m(H) = m(H₂O) × 2A(H)/M(H₂O) = m(H₂O) × 2.016/18.015 = m(H₂O) × 0.111907. Note the factor of 2: one water molecule carries two hydrogens.

STEP 3 — oxygen, by difference. m(O) = m(sample) − m(C) − m(H). This is not a measurement and it inherits the error of all three weighings.

STEP 4 — the empirical formula. Divide each element mass by its atomic weight to get moles, divide all three by the smallest, and multiply by the smallest whole number that turns every ratio into a whole number. A multiplier k from 1 to 12 is accepted when every scaled ratio is within max(0.05, 0.015 × nearest) of a whole number — a declared judgement, not a physical constant, chosen so that a 1.5 ratio is never rounded to 2.

ROUNDING STAGE. Element masses, moles and ratios are all carried at arbitrary decimal precision. Subscripts are rounded ONCE, after multiplication by k, never on the intermediate ratios. Every numeric output is rounded once more at the return boundary, to ten decimal places.

THE OXYGEN NOISE FLOOR. In C/H/O mode, a residual below 0.1 percent of the sample mass is treated as zero and omitted from the formula, and appears in the unexplained-mass output instead. That 0.1 percent is a declared judgement parameter: it is roughly a quarter of the ±0.4 percentage-point agreement conventionally demanded of a combustion analysis, so a residual smaller than it is weighing noise rather than oxygen. Without the floor, a genuine hydrocarbon would acquire a spurious trace of oxygen and a nonsensical formula.

INVALID DOMAIN. Zero or negative masses are refused. If the collected CO₂ and H₂O imply more carbon plus hydrogen than the sample weighed, the result is refused outright with an explicit message, because mass cannot be created — the usual causes are a mis-weighed sample or a trap that was not dry before the run. A small negative residual, up to 0.5 percent of the sample mass, is tolerated as weighing noise and surfaces as a negative unexplained-mass value.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Flowers, P., Theopold, K., Langley, R. & Robinson, W. R. (2019). Chemistry 2e, Section 3.2 'Determining Empirical and Molecular Formulas'. OpenStax, Rice University. Peer-reviewed, CC BY 4.0. PRIMARY SOURCE for the second half of this calculation: the procedure of converting element masses to moles, dividing by the smallest, and multiplying to whole-number subscripts, together with the definitions of empirical and molecular formula. Retrieved 2026-07-29. Open access.
2. [2]ASTM D3176-15, 'Standard Practice for Ultimate Analysis of Coal and Coke', ASTM International, West Conshohocken, PA. SECOND, INDEPENDENT AUTHORITY consulted for this page: it mandates precisely the treatment used here — there being no satisfactory direct ASTM test method for determining oxygen, oxygen shall be calculated by subtracting from 100 the sum of the other components of the ultimate analysis (carbon, hydrogen, nitrogen, sulfur and ash). It agrees with the by-difference method, and its inclusion of nitrogen, sulfur and ash in that subtraction is the reason this page is scoped to C/H/O compounds only. Retrieved 2026-07-29. Standards document, paywalled: the scope and the oxygen-by-difference clause are quoted from the publicly available scope summary.
3. [3]ASTM D5373-21, 'Standard Test Methods for Determination of Carbon, Hydrogen and Nitrogen in Analysis Samples of Coal and Carbon in Analysis Samples of Coal and Coke', ASTM International. The standardised instrumental form of this experiment: carbon dioxide, water vapour and elemental nitrogen in the combustion gas stream are determined by instrumental detection, giving carbon, hydrogen and nitrogen. Oxygen is not among the determinands, which corroborates why it must be taken by difference. Retrieved 2026-07-29. Standards document, paywalled: scope quoted from the publicly available summary.
4. [4]Meija, J. et al. (2021). 'Atomic weights of the elements 2021 (IUPAC Technical Report)'. Pure and Applied Chemistry 93(5), 573–600, doi:10.1515/pac-2019-0603, as maintained in the CIAAW table 'Abridged Standard Atomic Weights 2024' (named revision: 2024). The named, versioned source of the three atomic weights and therefore of both gravimetric factors: C 12.011, H 1.0080, O 15.999, giving M(CO2) = 44.009 and M(H2O) = 18.015 g/mol. Retrieved 2026-07-29. Open access.