Feed formulations are written in kilograms of formic acid and bought in bags of formate salts. Between those two statements sits a conversion that is fixed by chemistry and frequently done wrong: the acid equivalent of the salt. Get it wrong and a premix is either under-acidified or over-dosed, and the resulting complaint is usually about palatability rather than about pH.
This note gives the conversion for the common formate and formic acid products, explains why calcium formate is used rather than the free acid in dry feed, and lists what to check on a feed-grade certificate.
The formic acid equivalent table
The calculation is straightforward: each mole of salt delivers one or two moles of formic acid (HCOOH, 46.03 g/mol), divided by the molecular weight of the salt.
| Product | Formula | Molecular weight | Formic acid equivalent | Physical form |
|---|---|---|---|---|
| Formic acid | HCOOH | 46.03 | 100% | Corrosive liquid, pungent |
| Ammonium formate | HCOONH4 | 63.06 | 73.0% | Deliquescent solid or solution |
| Calcium formate | Ca(HCOO)2 | 130.11 | 70.75% | Free-flowing powder |
| Potassium diformate | KH(COOH)2 | 130.14 | 70.7% | Crystalline solid |
| Sodium formate | NaHCOO | 68.01 | 67.7% | Crystalline solid |
Two worked examples show how the table is used:
- A formulation calls for 0.50% formic acid equivalent. With calcium formate: 0.50 / 0.7075 = 0.71% of the complete feed.
- A formulation calls for 1.00% formic acid equivalent. With calcium formate: 1.00 / 0.7075 = 1.41% of the complete feed.
The same arithmetic applies when comparing quotations: a supplier quoting a formate salt per tonne cannot be compared with a supplier quoting free acid until both are converted to the same acid-equivalent basis. This is the single most common error in formate purchasing.
Why the salt rather than the acid
Free formic acid is the cheapest acid equivalent per kilogram and the least convenient product in the feed mill. It is corrosive to mild steel, volatile in the bag and in the mixer, and it reduces feed intake at effective doses because of its smell and taste. Dosing it safely means dedicated acid-proof equipment, and dosing it effectively means accepting a palatability penalty.
Calcium formate is a dry, near-neutral powder. It dissociates after ingestion, releasing the formate ion in the stomach where the acidifying effect is wanted, and it carries no free acid hazard in the mill. The buffering is the point: it delivers a larger acid load than the same weight of free acid could be dosed at, without the handling and palatability constraints.
Two secondary consequences follow, and both belong in the formulation sheet:
- Calcium contributes. Calcium formate is 30.8% calcium by formula. At 1.4% inclusion that is roughly 0.43% calcium in the complete feed, which must be deducted from the limestone or monocalcium phosphate allowance. Feed formulators who ignore this end up with an over-specified calcium level and an under-specified phosphorus level.
- Sodium versus calcium. Sodium formate at 67.7% acid equivalent carries 33.8% sodium, which matters in formulations where the sodium ceiling is tight. Calcium formate is the better choice where sodium is constrained; sodium formate is preferred where the calcium is already adequate.
What the acid does in the animal
The mechanism is pH, not pharmacology. A lower gastric pH favours pepsin activity, which has an acidic pH optimum, and disfavours acid-sensitive bacteria that pass through the stomach. That is why formate-based acidifiers are used most heavily in weaning diets, where the young animal’s own acid secretion is not yet fully developed and where the feed is buffered by the milk replacer and by high-protein ingredients.
Because the effect depends on how much acid actually arrives in the stomach, the acid equivalent – not the inclusion rate – is the number that predicts performance. Two products included at 1% with acid equivalents of 70.75% and 67.7% deliver 4.5% different acid loads, which is enough to change a trial result.
Claims about gut health, performance or pathogen reduction are regulated separately from the chemistry, and must match the authorised claim for the additive in the destination market. This note deliberately stops at the chemistry and the specification.
Reading a feed-grade certificate
| Line | Why it is there |
|---|---|
| Assay min 98% | Purity of the salt. The balance is moisture and calcium carbonate |
| Calcium min 30.1% | 98% assay expressed as calcium; a titratable cross-check on purity |
| Moisture max 0.5% | Caking and dust; also protects the assay |
| Arsenic max 5 ppm | The heavy-metal line feed auditors look for |
| Particle size | Typically 60-80 mesh for premix dispersion; coarse material settles out of a mash |
| Dioxin statement | Requested in the EU and in several Asian markets for feed additives |
| GMP+ / FAMI-QS certificate | The chain-of-custody document for feed-grade material |
The cross-contamination question
Calcium formate is made on the same type of plant as the industrial and concrete grades, so a feed buyer should ask a direct question: how is feed-grade material segregated from technical-grade material – dedicated line, dedicated dryer, or campaign production with validated cleaning? The answer belongs in the supplier qualification file, not in a marketing brochure. Feed auditors ask for it, and “we always wash the line” is not a documented answer.
Inclusion rates in practice
Typical inclusion for calcium formate runs from about 0.5% to 2% of the complete feed, with the higher end in weaning diets for piglets and in pressure situations in poultry. Three constraints set the ceiling:
- The calcium contribution, as described above, which competes with other calcium sources in the mineral allowance.
- Palatability, which falls off at high inclusion even with the salt form – the acid load eventually registers.
- The national maximum for the additive, which differs between markets and is a legal limit, not a technical one.
Product page and related reading
Our feed, concrete and industrial grades are specified on the calcium formate (CAS 544-17-2) product page. The sodium salt is described on the sodium formate page, and our guide to sodium bicarbonate in animal feed covers the rumen-buffer side of the same formulation problem.
Disclaimer. Acid equivalents and calcium contents are calculated from molecular formulas with the working shown. Inclusion rates are indicative starting points for trials, not recommendations: the legal maximum, the authorised claim and the nutritional balance of the complete feed govern what may be used and what may be said about it.
For a feed-grade COA, a GMP+ certificate copy, or a 1 kg sample for a trial mix, contact PolyBlueChem.
Related reading.