This free SAR calculator works out the sodium adsorption ratio of irrigation water from a standard lab report. Enter sodium, calcium and magnesium from your water analysis to calculate the sodium adsorption ratio (SAR). Add the electrical conductivity to also get the salinity class and the FAO infiltration risk, which together show whether the water is safe to use on your soil and crops.
How to use the SAR Calculator
- Select whether your results are in mg/L or meq/L.
- Enter sodium (Na⁺), calcium (Ca²⁺) and magnesium (Mg²⁺).
- Add electrical conductivity (EC) if you have it, to get the salinity class and infiltration risk.
- Press Calculate SAR to see the value and the hazard class.
The SAR calculator converts mg/L to meq/L for you, so you can enter results straight from the report. Infiltration guidance follows FAO’s Water Quality for Agriculture (Ayers and Westcot).
SAR formula
Sodium hazard classes (US Salinity Laboratory, Handbook 60)
| SAR | Class | Meaning |
|---|---|---|
| < 10 | S1 – Low | Safe for most soils and crops |
| 10 – 18 | S2 – Medium | Risk on fine-textured soils with low leaching |
| 18 – 26 | S3 – High | Harmful on most soils; needs gypsum and good drainage |
| > 26 | S4 – Very high | Generally unsuitable for irrigation |
Salinity classes by EC
| EC (µS/cm) | Class |
|---|---|
| < 250 | C1 – Low salinity |
| 250 – 750 | C2 – Medium |
| 750 – 2,250 | C3 – High |
| > 2,250 | C4 – Very high |
SAR and salinity interact. Very pure water (low EC) with even a moderate SAR can seal the soil surface, while saline water keeps soil aggregated. That is why the FAO infiltration assessment looks at both together.
Frequently asked questions
What is a good SAR value for irrigation water?
An SAR below 3 is ideal and below 10 is generally safe. Between 10 and 18 needs care on clay soils, and above 18 usually requires gypsum and good drainage.
Can I enter my lab results in mg/L?
Yes. Choose mg/L and the calculator converts sodium, calcium and magnesium to meq/L using their equivalent weights.
What is adjusted SAR?
Adjusted SAR corrects for calcium lost as carbonate precipitate in waters with high bicarbonate. FAO now recommends the adjusted Ca value (Ca_x) method instead of the older adj SAR. This calculator gives the standard (practical) SAR.
How do I lower SAR?
Add calcium, usually as gypsum applied to the soil or injected into the water. Blending with low-sodium water and increasing leaching also help.
What is the sodium adsorption ratio formula?
SAR = Na ÷ √((Ca + Mg) ÷ 2), with all three concentrations in meq/L.
How do you calculate SAR by hand?
Convert mg/L to meq/L by dividing by 22.99 for Na, 20.04 for Ca and 12.15 for Mg. For Na 230 mg/L, Ca 80 mg/L and Mg 24 mg/L you get about 10.0, 4.0 and 2.0 meq/L, so SAR = 10.0 ÷ √3.0 = 5.8.
Is SAR the same as ESP?
No. SAR describes the water, while the exchangeable sodium percentage (ESP) describes the soil. Over time, soil irrigated with high-SAR water builds up sodium unless it is managed with gypsum and leaching.
Related tools
For guidance only. Local soil conditions, rainfall and crop tolerance matter. Consult an agronomist for irrigation decisions. Built by an accredited water-testing laboratory professional.