How to use this calculator
- Fill at the target. Concentrate = volume × %/100, added to water (not the reverse), ideally through a venturi mixer.
- Verify with the refractometer. Reading × the data-sheet factor = actual %. The screen shows the reading a correct mix gives.
- Correct lean or rich. Lean: add the computed concentrate. Rich: cut with the computed water and re-read after mixing.
- Top off leaner than target. Evaporation drifts sumps rich — many shops top off at half strength and let the refractometer arbitrate.
How it works
Coolant concentration is percent of the total mix, which makes every mixing question one line of algebra:
fill: V_c = V·c/100 · raise: ΔC = V·(c_t − c_a)/(100 − c_t) · cut: ΔW = V·(c_a − c_t)/c_t
The refractometer closes the loop: actual % = Brix reading × the coolant's factor, so the screen also reports the reading a correctly mixed sump should show. Concentration is the cheapest tooling variable in the shop — it moves surface finish, tool life and rust together. The cutting side of the same setup lives in the speeds & feeds calculator and the machining cost calculator prices what bad coolant quietly costs.
Worked example
Verified against the live calculator
A 50 gal sump filled to 8%, on a coolant
whose refractometer factor is 1.5:
concentrate = 50 × 0.08 = 4 gal · water = 46 gal · expected reading = 8 ÷ 1.5 ≈ 5.3 Brix
A week later the refractometer reads 4.0 — that is 4.0 × 1.5 = 6% actual, lean. Raising 50 gallons back to 8% takes 50 × (8 − 6)/(100 − 8) ≈ 1.1 gal of concentrate. Had it read 8.0 (12% actual, rich from evaporation), the cut would be 50 × (12 − 8)/8 = 25 gallons of water.
Frequently asked questions
How much coolant concentrate per gallon of water?
Work from the total: concentrate = volume × target% ÷ 100. A 50-gallon sump at 8% takes 4 gallons of concentrate and 46 of water — an 11.5:1 water-to-concentrate ratio. The "per gallon of water" phrasing hides that the percentage is of the total mix.
How do you read coolant concentration with a refractometer?
Put a drop of well-mixed, skimmed sump fluid on the prism and read the Brix line — then multiply by the coolant’s refractometer factor from its data sheet (typically 1.0–2.5). A reading of 4 on a factor-1.5 semi-synthetic means 6% actual, not 4%.
How do you raise coolant concentration in a running sump?
Add concentrate ΔC = V × (target − actual) ÷ (100 − target). A 50-gallon sump at 6% needs about 1.1 gallons of concentrate to reach 8% — trickled in while the pump circulates, never dumped in one spot.
Why does coolant concentration creep up over time?
Evaporation takes only water; the concentrate stays. Every top-off at full strength ratchets the sump richer, which is why shops top off at half strength or leaner and re-check with the refractometer rather than trusting the mix that went in.
Method & assumptions
- Volumes treated as additive (fine at metalworking concentrations); percentages are by volume of total mix, the shop and TDS convention.
- The refractometer factor is the coolant maker's number — no factor table is embedded; 1.0 gives the straight Brix relation.
- Tramp oil, dissolved soaps and emulsion instability all inflate Brix readings; skim and mix before reading.
- Running band, water hardness limits and biocide schedule come from the coolant data sheet — this screen does the arithmetic, not the chemistry.