DEF usage and cost calculator

This DEF usage and cost calculator returns how much diesel your truck burns, how much diesel exhaust fluid it uses, what that DEF costs, and the DEF cost per mile.

Diesel exhaust fluid is a small cost next to fuel, but it is a real one, and it is easy to leave out when pricing loads. Trucks with selective catalytic reduction (SCR) systems inject DEF into the exhaust to cut nitrogen oxide emissions. The amount they use is tied to how much diesel they burn.

How it works

• Diesel gallons = miles / MPG
• DEF gallons = diesel gallons x dosing rate
• DEF cost = DEF gallons x DEF price
• DEF cost per mile = DEF cost / miles

Enter miles for one trip, a month or a year. The default dosing rate is 2.5%, in the middle of the 2 to 3% often quoted for modern engines. Your engine maker sets the real number, and your own DEF fill-ups are the best check.

A worked example

A truck runs 10,000 miles a month at 6.5 MPG, burning about 1,538 gallons of diesel. At a 2.5% dosing rate, it uses about 38 gallons of DEF. At $3.50 a gallon, that is about $135 a month, or about 1.3 cents per mile.

Small next to fuel, but over 120,000 miles a year it adds up to more than $1,600. If you buy boxed DEF at a higher price, it can be noticeably more.

How much DEF does a truck use in practice

The dosing rate is not fixed. It changes with engine load, exhaust temperature and the engine's calibration. Heavy loads, steep grades and hot weather can push it up; light loads and easy highway running can bring it down. Track DEF fill-ups the same way you track fuel for a month and you will know your truck's real rate.

Our guide to DEF cost and usage per gallon of diesel covers dosing, storage, shelf life, contamination and what to do when the system throws a fault.

Buying and storing DEF

• Bulk from the pump is usually the cheapest per gallon at truck stops.
• Boxed jugs cost more but are handy for top-ups and remote areas.
• Storage matters. DEF degrades with heat and time and can freeze in cold weather. Keep spare jugs out of direct sun.
• Use certified fluid. Look for fluid that meets ISO 22241. Contaminated DEF can damage the SCR system.
• Never mix in water or other fluids. It is a recipe for an expensive repair.

Idling and DEF

Idling burns diesel, and some DEF along with it, without adding miles. That raises both the fuel and the DEF cost per mile. The idle cost calculator shows what idle time costs, and the guide to semi truck APU cost covers one common alternative for climate control while parked.

Putting it in the rate

DEF belongs in your cost per mile next to fuel. For a whole trip, the trip fuel cost calculator prices the diesel; add the DEF line from here.

How much DEF does a truck hold

DEF tanks are sized so a truck can go several diesel fill-ups between DEF fills. Capacity varies widely by truck and configuration, so check the spec sheet or the label near the filler. A simple habit is to top up DEF every time you fuel, or every second time, so the level never runs low on the road. Keep a sealed jug in the truck for the times a pump is out of service. Running the tank dry triggers warnings and, eventually, reduced engine power.

Emissions systems and the law

SCR, the DPF and the rest of the aftertreatment system are required emissions equipment. Removing or disabling them, or using devices that defeat them, is illegal under federal law and can bring heavy penalties. If the system is causing problems, the answer is a diagnosis and a repair by a qualified technician.

Where we fit

When we dispatch a truck, every load gets priced against your floor rate, with fuel and fluids in it. We find and negotiate the load, you say yes, counter or no, and the broker sends the rate confirmation to you to sign. How this works for one truck is on our owner-operators page.