Turbidity to TSS Estimator
Convert a turbidity reading in NTU to an estimated total suspended solids (TSS) concentration in mg/L using a site-calibrated linear correlation TSS = a·turbidity + b. Enter your turbidity and, ideally, your own regression slope and intercept for a meaningful estimate.
Enter Values
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How to use this calculator
- Enter the measured turbidity in NTU (nephelometric turbidity units) from your probe or lab.
- Enter the slope a (mg/L of TSS per NTU) and intercept b (mg/L) from a regression of paired lab TSS against turbidity for your site; if you leave them blank the tool uses indicative defaults of a = 1.1 and b = 0.
- Read the estimated TSS and the correlation equation used, and treat the result as approximate unless a and b are your own calibrated values.
How it works
The tool applies a straight-line relationship, TSS = a·turbidity + b, where a is the slope (mg/L per NTU) and b is the intercept (mg/L). This assumes the particles causing turbidity are consistent in size and type, so that scattered light scales linearly with suspended mass. The slope and intercept must be established for each water body by laboratory regression, because the same NTU can correspond to very different TSS depending on the sediment.
Worked example
Worked example. A turbidity of 50 NTU with the default slope a = 1.1 and intercept b = 0 gives TSS = 1.1 × 50 + 0 = 55 mg/L. With a site-calibrated slope of 1.3 and intercept of 5, a reading of 120 NTU gives TSS = 1.3 × 120 + 5 = 161 mg/L.
Common mistakes
- Using the default a and b as if they were universal — they are only indicative; an un-calibrated correlation can be wrong by a factor of two or more.
- Assuming turbidity and TSS are the same quantity; turbidity is an optical light-scattering measure while TSS is a gravimetric dry mass, and their ratio changes with particle type.
- Applying a correlation fitted to one flow regime (e.g. baseflow) to very different conditions such as a storm event, where the particle mix and the a/b relationship shift.
Frequently asked questions
Why can't one universal NTU-to-mg/L factor be used?
Turbidity depends on how particles scatter light, which varies with their size, shape, colour and composition. Fine clay scatters far more light per unit mass than coarse sand, so the same TSS can read very different turbidities. That is why the slope a (and often the intercept b) must be calibrated separately for each site and sometimes each season.
How do I calibrate my own a and b?
Collect a set of grab samples across a range of turbidities, measure the field turbidity and the laboratory gravimetric TSS for each, then fit a linear regression (TSS on turbidity). The regression slope is a and the intercept is b. Aim for a good spread of values and a strong correlation coefficient before trusting the fit.
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Tip: Enter any known values to calculate the remaining results.
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