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How to Check Coolant Concentration
Guide · Updated October 28, 2026
Water-soluble coolant works only in its concentration window. Too lean and you get rust, poor tool life and bacteria; too rich and you get foaming, sticky residue and skin irritation. A handheld refractometer checks it in seconds.
Speeds and feeds too
Pair good coolant with the right RPM and feed in the free speeds and feeds calculator.
Step by step
- Calibrate: Put a few drops of clean water on the prism and set the line to zero.
- Take a sample: Pull coolant from the sump while it is circulating, away from tramp oil on top.
- Read Brix: Place drops on the prism, close the cover, look toward light and read where the light/dark line crosses the Brix scale.
- Apply the factor: Concentration % = Brix reading × the coolant's refractometer factor from the data sheet (often 1.0 to 2.0).
- Adjust: Add premix at a richer ratio to raise concentration, or water to lower it. Never add straight concentrate to the sump.
- Log it: Record date, reading and top-ups so you see trends.
Worked example
Brix reads 4.0, factor 1.5 → 6% concentration. Target is 8%. For a 50 gal sump, you need about 1 gal more concentrate (2% × 50 gal); add it as a rich premix (e.g., 15%) rather than neat.
Typical targets
General milling and turning: 5–8%. Grinding: 3–5%. Heavy tapping or broaching: 8–12%. Always follow the coolant maker's recommendation.
Fuzzy line?
A blurry line usually means tramp oil contamination. Skim the sump; a blurry line reads high.
Tips
- Read cutting fluid and coolant basics.
- Check pH too; most coolants want about 8.5–9.5.
- Rich coolant for tapping; leaner for grinding.
- Wear gloves; see the product's safety data sheet.
FAQ
How do you check coolant concentration?
Read a sample on a refractometer's Brix scale and multiply by the coolant's refractometer factor from its data sheet.
What concentration should machine coolant be?
Commonly 5–8% for general machining, 3–5% for grinding and higher for tapping, but follow the manufacturer's range.
What is a refractometer factor?
A multiplier, specific to each coolant, that converts the Brix reading to actual concentration.
How do I raise coolant concentration?
Add a premix richer than your target. Avoid pouring straight concentrate into the sump.
How often should I check coolant?
Daily or at least weekly for busy machines, since water evaporates and concentration climbs.
Related guides and calculators
- Cutting fluid and coolant basics
- Speeds and feeds calculator
- Milling speeds and feeds
- Turning speeds and feeds
- Tapping speeds and feeds
- Grinding speeds and feeds
- Shop Safety Basics for Machinists
- How to Protect Finished Parts and Gages from Rust
- How to Degrease and Clean Machined Parts
- All free machinist calculators
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Questions? Ask a machinist.
General shop information only — follow your coolant maker's data sheet and safety data sheet.