Calculate diesel exhaust fluid (DEF) consumption from miles driven or fuel burned, plus DEF cost and how far a full DEF tank will last.
DEF Consumption Formula
DEF = D * (r / 100)
US: D (US gal) = M (mi) / MPG Metric: D (L) = M (km) * C (L/100 km) / 100
US: Range (mi) = (T (US gal) * 100 / r) * MPG Metric: Range (km) = (T (L) * 100 / r) / C * 100
- DEF is the diesel exhaust fluid consumed (gallons or liters)
- D is the diesel fuel burned (gallons or liters)
- r is the DEF dose rate by volume as a percent of diesel burned; blank uses an illustrative 3%, not a universal engine setting.
- M is the distance driven (miles or kilometers)
- MPG is US miles per gallon. C is fuel consumption in liters per 100 km; these are reciprocal measures.
- T is the DEF tank capacity (gallons or liters)
The calculator has three modes built on these formulas. The distance mode first converts miles driven into diesel burned using your fuel economy, then applies the dose rate to get DEF used. The fuel mode skips the first step and applies the dose rate directly to the diesel volume you enter. The tank range mode inverts the formula: it works out how much diesel can be burned before a full DEF tank runs dry, then converts that diesel into distance using your fuel economy. The advanced options multiply distance or fuel by a vehicle count for fleet estimates and apply a DEF price to estimate cost.
Typical DEF Dose Rates and Shelf Life
The dose rate is set by the engine’s SCR system, not by this calculator. Use your measured consumption or model-specific manufacturer guidance. Cummins describes an average of 3 to 5% for its equipment and emphasizes that use varies with environment, operation and duty cycle. The calculator’s blank 3% value is only a planning assumption.
| Illustrative dose | Equivalent volume ratio |
|---|---|
| 2% | 1 volume DEF per 50 volumes diesel |
| 3% | 1 per approximately 33.33 |
| 5% | 1 per 20 |
DEF quality depends on storage conditions. The constant-temperature shelf-life guidance below is reproduced in a Scania operator manual citing ISO 22241-3. Count storage age from manufacture and follow the supplier’s expiry, handling and testing instructions; heat exposure can shorten useful life.
| Constant storage temperature | Expected DEF shelf life |
|---|---|
| At or below 50 F (10 C) | 36 months |
| At or below 77 F (25 C) | 18 months |
| At or below 86 F (30 C) | 12 months |
| At or below 95 F (35 C) | 6 months |
| Above 95 F (35 C) | Test before use |
Example Problems
Example 1: Annual DEF use for a semi truck. A truck runs 120,000 miles per year at 6.5 MPG with a 3% dose rate. Diesel burned is 120,000 / 6.5 = 18,461.54 gallons. DEF used is 18,461.54 * 0.03 = 553.85 gallons, or about 217 miles per gallon of DEF. At $12.50 per gallon of DEF, that is roughly $6,923 per year.
Example 2: DEF tank range for a pickup. A diesel pickup has a 5 gallon DEF tank, gets 20 MPG, and doses at 2%. Diesel burned per DEF tank is 5 * 100 / 2 = 250 gallons. The theoretical range is 250 * 20 = 5,000 miles per full DEF tank. An illustrative point with 10% remaining is 4,500 miles; neither number guarantees a safe refill interval. Follow the vehicle’s warnings and manufacturer refill instructions.
FAQ
How much DEF does a truck use per gallon of diesel? It depends on the engine and operating conditions. A 3% volume rate means 0.03 gallon DEF per gallon diesel, or 1 gallon per approximately 33.33 gallons diesel. Use measured or manufacturer-specific consumption rather than treating a category average as a guarantee.
What happens if the DEF tank runs empty? Warnings, power reductions and operating restrictions depend on the vehicle. Follow its operator manual and refill warnings. The calculator retains an illustrative point with 10% remaining; this is a planning comparison, not manufacturer advice or a guarantee against running out.
Does using more DEF mean worse fuel economy? DEF and diesel use depend on engine operation, emissions calibration and duty cycle. This calculator applies the rate and fuel economy you enter; it does not diagnose engine condition or predict fuel savings. Check unexplained consumption changes using the vehicle manufacturer’s guidance.
