How to use this calculator
Start by entering the number of miles you drive per month. This should include all miles, both loaded and deadhead. Next, set your DEF consumption rate. The default of 2.5 gallons per 1,000 miles works well for most Class 8 trucks, but you can adjust it based on your engine and driving conditions.
Enter your DEF tank size in gallons and the price you typically pay per gallon. The calculator will instantly show your monthly and annual DEF usage, total costs, cost per mile, and how often you need to refill your DEF tank.
What is DEF and why do trucks need it?
Diesel Exhaust Fluid (DEF) is a solution of 32.5% urea and 67.5% deionized water. It is injected into the exhaust stream of diesel engines equipped with Selective Catalytic Reduction (SCR) systems. The urea reacts with nitrogen oxides (NOx) in the exhaust, converting them into harmless nitrogen and water vapor.
All diesel trucks manufactured after 2010 are required to have SCR systems to meet EPA emissions standards. DEF is not a fuel additive. It is stored in a separate tank and consumed automatically by the SCR system. Running out of DEF or using contaminated fluid can trigger engine derating and lead to expensive repairs.
Typical DEF consumption rates
DEF consumption is proportional to diesel fuel usage. Most trucks consume DEF at a rate of about 2% to 3% of their total diesel consumption. For a truck that burns 6 miles per gallon of diesel, this translates to roughly 2 to 3 gallons of DEF per 1,000 miles driven.
Several factors affect your actual DEF consumption rate. Heavier loads, frequent stop and go driving, steep terrain, and older SCR systems tend to increase DEF usage. Highway cruising at steady speeds with moderate loads will keep consumption closer to the lower end of the range. Tracking your actual usage over a few months will give you the most accurate rate to enter into this calculator.
How to save on DEF costs
The biggest savings come from where you buy DEF. Truck stop pumps are the most convenient but also the most expensive option, often charging two to three times the price of retail jugs. Buying 2.5 gallon containers from auto parts stores, big box retailers, or farm supply shops can cut your per gallon cost significantly.
If you have a home base or terminal, consider purchasing DEF in 55 gallon drums or 275 gallon totes from industrial suppliers. The per gallon price drops even further at these volumes. Just make sure you store it properly: keep it sealed, out of direct sunlight, and at temperatures below 86F to prevent degradation.
Frequently asked questions
How much DEF does a truck use per mile?
Most diesel trucks use about 2 to 3 gallons of DEF per 1,000 miles, which equals roughly $0.005 to $0.01 per mile at retail prices. This is a small fraction of your total operating cost but adds up over a full year of driving.
How much does DEF cost per gallon?
Prices vary significantly by source. Truck stop pumps charge $5 to $8 per gallon, while retail jugs at auto parts stores or big box retailers cost $2 to $3 per gallon. Bulk purchasing from industrial suppliers can bring the cost below $2 per gallon.
What happens if you run out of DEF?
Your truck will progressively reduce engine power and eventually limit your speed to as low as 5 mph. This is mandated by EPA regulations and cannot be disabled. Always keep your DEF tank at least a quarter full to avoid unexpected derating on the road.
Does DEF go bad?
DEF has a shelf life of approximately 12 months when stored properly between 12F and 86F. High temperatures and UV exposure accelerate degradation. Always check the manufacture date on jugs and avoid buying from retailers with slow inventory turnover.