This coolant concentration calculator turns a refractometer reading into percent by multiplying the Brix value by the factor on the product data sheet, then sizes the concentrate and water for a new charge, a top-up or a rich sump in liters and US gallons. It applies to water-mix metalworking fluids.
Percent is by volume of concentrate in the diluted fluid, with 1 US gal = 3.785411784 L. Modes 3 and 4 need the current fluid volume, not only the sump capacity.
Coolant concentration calculator
Enter the values above; the result appears here.
Tank-side measurements resolve to about ±0.25 % (one supplier's technical bulletin), so read the result to one decimal.
Results are arithmetic from your inputs. The refractometer factor and the target concentration come from the data sheet of the product in your sump.
Standards, regulations and exposure limits are checked against their issuing bodies, product values against the named data sheet, and every figure carries its source and date; see our methodology and fact-checking.
How Does the Coolant Concentration Calculator Work?#
The coolant concentration calculator works in four modes: it converts a Brix reading to percent, mixes a new charge, sizes a top-up with its premix strength, and computes how much fluid to drain from a rich sump before adding water.
This calculator groups the arithmetic into four modes and is one of the calculators and request tools here. The table compares the modes by use, inputs and outputs.
| Mode | Use it when | Inputs | Outputs |
|---|---|---|---|
| 1 Reading to percent | a Brix reading needs converting | Brix, factor; optional titration drops and factor | concentration in % by volume |
| 2 New charge | a clean sump is filled | total volume, target % | concentrate and water volumes, ratio |
| 3 Top-up | the level has dropped | sump capacity, current volume, current %, target % | concentrate and water volumes, premix strength |
| 4 Rich sump | the top-up result is below zero | sump capacity, current volume, current %, target % | drain volume, refill water |
Percent is by volume of concentrate in the use dilution; volumes in liters with US gallons.
Mode 1: refractometer reading to percent#
Mode 1 multiplies the Brix reading by the product factor and, as an optional cross-check, multiplies titration drops by the titration factor. Tank-side readings resolve to about ±0.25 % in one supplier's technical bulletin.
C = B x f and Ct = drops x titration factor
Mode 2: new charge from concentrate and water#
A new charge needs Vc = Vt x Ct / 100 of concentrate and the rest water, so 400 L (105.7 US gal) at 6 % takes 24 L (6.34 US gal) of concentrate and 376 L (99.33 US gal) of water.
Vc = Vt x Ct / 100 and Vw = Vt - Vc
Mode 3: top-up volume and premix strength#
A top-up restores the target concentration at full volume from the current volume and measured concentration, and the calculator reports the premix strength Cm. HSE MW5 (03/21) says to top up with a premix below the recommended concentration.
Vc = (Ct x Vt - C1 x V1) / 100, Vw = (Vt - V1) - Vc, Cm = (Ct x Vt - C1 x V1) / (Vt - V1)
Mode 4: rich sump drain-down#
A sump is rich when Vc comes out below zero, and the calculator then gives the drain volume D and the water to refill. A 400 L sump holding 380 L at 10 %, target 7 %, drains 100 L (26.4 US gal) and refills with 120 L (31.7 US gal) of water. A rich sump with odor or sludge needs sump cleaning and recharge, not dilution.
D = V1 - (Ct x Vt / C1) and W = Vt - (V1 - D)
How to Read the Refractometer for the Calculator#
Read the refractometer for the calculator by zeroing it on the same water used for mixing at about 20 °C (68 °F), placing a few drops of circulating fluid on the prism and taking the clear boundary line at the bottom of any haze as the Brix value. The UKLA/HSE Good Practice Guide v2.5 (February 2025) sets this zero.
- Zero the refractometer on the mix water at about 20 °C (68 °F).
- Place a few drops of circulating fluid on the prism.
- Read the clear boundary line at the bottom of any haze.
- Enter the Brix value and the data sheet factor in mode 1.
- Record the result with the date and machine.
Why the refractometer factor changes the result#
The same 4.5 °Brix reading gives 9.0 % with factor 2.0 and 4.5 % with factor 1.0, so the factor field is required. One supplier's bulletin reports factors from about 0.9 for emulsions to 3.4 for some synthetics.
Step-by-step refractometer readings and correction factors from the data sheet are explained in the concentration guide.
Worked Example: From a Brix Reading to a Top-Up#
A 400 L (105.7 US gal) sump holding 300 L (79.3 US gal) reads 4.5 °Brix with a product factor of 2.0, so it runs at 9.0 %, and bringing it to 7 % at full volume takes 1 L (0.26 US gal) of concentrate and 99 L (26.2 US gal) of water, a 1 % premix.
- Convert the reading: 4.5 °Brix x factor 2.0 = 9.0 %.
- Enter mode 3: capacity 400 L, current 300 L at 9.0 %, target 7 %.
- Read the result: 1 L of concentrate, 99 L of water.
- Pour the concentrate into the water, then add the premix to the sump.
The premix is weaker than the sump because evaporation concentrated it.
What Limits the Calculator Result?#
The calculator result is only as good as the Brix reading, and tramp oil, temperature deviation and a change in emulsion droplet size distribution all distort that reading before any arithmetic starts. Tramp oil makes readings drift higher than actual. Sources and removal are covered under tramp oil in coolant.
BGR/GUV-R 143 (2009), the predecessor of DGUV Regel 109-003, names temperature and droplet size as further error sources. Titration ignores tramp oil but is biased by alkaline builders, acids and biocides. If the two results differ greatly, DGUV Regel 109-003 calls for a laboratory method.
Where Does the Target Concentration Come From?#
The target concentration comes from the product data sheet, and an STLE webinar (April 2022) gives 5 to 10 % concentrate by volume as the in-use range.
Sourced coolant concentration targets by fluid type are listed in the concentration guide.
Coolant Calculator FAQ#
These four answers cover the reading, ratio, overdose and rich sump.
Is the Brix reading the coolant percentage?#
Only when the data sheet factor is 1.0. Otherwise the percent is the Brix reading multiplied by the factor.
How much water do you mix with 1 liter of coolant concentrate?#
At 5 % by volume, 1 L of concentrate goes into 19 L of water, and at 10 % into 9 L of water. Pour the concentrate into the water.
What happens if you put too much coolant concentrate in the sump?#
Too much concentrate causes dermatitis, foaming and heavy residues, according to the OSHA Metalworking Fluids manual.
Causes of foam beyond concentration are on coolant foaming.
How do you lower a coolant concentration that is too high?#
Lower it with water or a very weak premix, and drain part of the fluid first when the sump has no room.