DEF Consumption
Calculator
Calculate exactly how much Diesel Exhaust Fluid your fleet uses per day, week, and month — and what it's costing you. Works for any fleet size.
What is DEF? Diesel Exhaust Fluid (DEF) is a solution of urea and deionized water used in Selective Catalytic Reduction (SCR) systems on all 2010+ diesel engines. EPA regulations require it — running out disables your vehicle. Most diesel engines consume DEF at 2–8% of fuel consumption depending on engine load and type.
DEF Consumption Calculator
Enter your fleet details below — results update instantly
| Vehicle / Equipment Type | DEF rate | Quantity |
|---|---|---|
Semi / Class 8 Truck ~150 gal diesel/day @ 5% DEF | 5% | 0 |
Medium-Duty Truck (Class 6–7) ~60 gal diesel/day @ 4% DEF | 4% | 0 |
Pickup / Light Diesel (Class 3–5) ~25 gal diesel/day @ 3% DEF | 3% | 0 |
Construction Equipment (Tier 4) ~40 gal diesel/day @ 4% DEF | 4% | 0 |
Generator (Tier 4 / EPA) ~20 gal diesel/day @ 5% DEF | 5% | 0 |
Bus / Coach ~80 gal diesel/day @ 6% DEF | 6% | 0 |
Agricultural Equipment ~50 gal diesel/day @ 4% DEF | 4% | 0 |
How much DEF does a diesel engine use?
Diesel Exhaust Fluid (DEF) consumption is directly tied to diesel fuel consumption. Most modern diesel engines with Selective Catalytic Reduction (SCR) systems use DEF at a rate of 2–8% of diesel consumption, depending on the engine type, load level, and operating conditions. A semi-truck burning 150 gallons of diesel per day will typically consume 6–9 gallons of DEF in the same period.
The EPA mandated SCR systems on all heavy-duty diesel engines beginning with 2010 model year vehicles as part of the federal emissions standards. SCR technology injects DEF into the exhaust stream, where it reacts with nitrogen oxides (NOx) to convert them into harmless nitrogen and water vapor. Without DEF, this system cannot function, and the engine control unit will progressively derate engine power — eventually preventing the vehicle from operating.
Lower DEF consumption
Vehicles operating at light loads or in stop-and-go conditions typically consume DEF at the lower end of the range — around 2–3% of diesel use — because less NOx is produced at partial load.
4–5% is the benchmark
Most commercial fleet managers plan around 4–5% as a working average across mixed operating conditions. This is the industry standard for budgeting and inventory planning purposes.
Up to 8% consumption
High-load operations — long-haul highways, heavy construction equipment, and buses running at capacity — can push DEF consumption up to 6–8% of diesel use. Plan your inventory accordingly.
Cold weather increases use
DEF consumption increases in cold weather as SCR systems work harder to maintain reaction temperatures. In extreme cold, DEF can also freeze (at 12°F / -11°C), which can disable the SCR system.
DEF consumption by vehicle type — complete reference table
Use this table to quickly estimate DEF usage for your specific fleet composition. All figures assume standard operating conditions at 80% average load.
| Vehicle / Equipment | Typical diesel/day | DEF rate | DEF gal/day | DEF gal/month | DEF cost/month ($3.25/gal) |
|---|---|---|---|---|---|
| Semi / Class 8 truck | 150 gal | 5% | 7.5 gal | 165 gal | $536 |
| Medium-duty (Class 6–7) | 60 gal | 4% | 2.4 gal | 53 gal | $172 |
| Light diesel pickup (Class 3–5) | 25 gal | 3% | 0.75 gal | 17 gal | $55 |
| Construction equipment (Tier 4) | 40 gal | 4% | 1.6 gal | 35 gal | $114 |
| Generator (Tier 4 / EPA) | 20 gal | 5% | 1.0 gal | 22 gal | $72 |
| Bus / coach | 80 gal | 6% | 4.8 gal | 106 gal | $345 |
| Agricultural equipment | 50 gal | 4% | 2.0 gal | 44 gal | $143 |
How to plan your fleet's DEF inventory
The biggest DEF-related mistake fleet managers make is treating DEF as an afterthought — ordering it reactively when a warning light appears rather than managing it proactively alongside diesel. The consequences of running dry are severe enough that DEF inventory should be planned with the same discipline as your fuel supply.
The right reorder point for DEF is when your supply drops to 25% of your storage capacity — the same rule of thumb used for diesel tanks. For a fleet running 10 Class 8 trucks 5 days per week, that's approximately 75 gallons of daily DEF consumption, or 375 gallons per week. A 330-gallon tote would need replacement every 4–5 days. A 1,000-gallon tank would need a refill roughly every 2 weeks.
Bulk DEF delivery from FuelGo is available in 275-gallon and 330-gallon totes, or by dedicated DEF tanker for high-volume operations. Bulk pricing is significantly lower than retail jugs — a 2.5-gallon jug from a truck stop typically costs $10–$15 (equivalent to $4–$6 per gallon), while bulk delivered DEF averages $2.50–$3.50 per gallon. For a fleet consuming 300 gallons per month, that's a potential saving of $300–$1,000 per month versus retail purchasing.
DEF storage & handlingDEF storage requirements every fleet manager should know
DEF has specific storage requirements that differentiate it from diesel. It must be stored in HDPE plastic containers — never in metal tanks, as DEF is corrosive to most metals including steel, copper, and aluminum. It should be kept between 12°F and 86°F (-11°C and 30°C) for extended storage. Above 86°F, DEF begins to degrade and lose potency over time.
Shelf life is typically 12 months when stored correctly — but degraded DEF can damage SCR systems, which are expensive to repair. If you're storing bulk DEF, rotate your stock, keep it out of direct sunlight, and check the expiration date on each delivery. FuelGo's DEF delivery includes documentation and quality certification with every order.
Generator Maintenance
Checklist
The complete commercial generator maintenance checklist — daily, weekly, monthly, and annual. Keep your generator running at peak performance and avoid costly breakdowns.
Complete Generator Maintenance Checklist
Click each item to mark it complete. Your progress is saved automatically. When finished, use the print button to save or print a PDF for your records. This checklist follows NFPA 110 guidelines for emergency and standby power systems.
Why generator maintenance is non-negotiable for commercial operations
A generator that fails during a power outage isn't just an inconvenience — for commercial operations, it can mean hundreds of thousands of dollars in lost production, spoiled inventory, regulatory violations, and in critical facilities like hospitals and data centers, genuine risk to human life. The vast majority of generator failures during outages are preventable with consistent, documented maintenance. Studies by the Electric Power Research Institute found that over 70% of generator failures during actual outages were caused by inadequate maintenance — not equipment age or mechanical defects.
The checklist above follows the maintenance intervals established by NFPA 110 (Standard for Emergency and Standby Power Systems), which is the governing standard for backup power systems in the United States. Compliance with NFPA 110 is required for hospitals, data centers, emergency services facilities, and any commercial building covered by local fire codes.
Running low on fuel, contaminated diesel, and clogged fuel filters account for the largest share of generator failures during real outages. This checklist addresses fuel system maintenance at every interval — but the most reliable protection is a scheduled on-site fuel delivery contract that keeps your tanks above 75% at all times.
Before every operation
Daily checks take under 10 minutes and catch the issues most likely to cause immediate failure — low oil, low fuel, and battery problems.
Test run + visual inspection
Weekly test runs prevent wet stacking in diesel generators and confirm that automatic transfer switches work when called upon.
Load testing required
NFPA 110 requires monthly load testing at minimum 30% rated capacity. Running diesel generators at low load causes wet stacking and premature failure.
Full professional service
Annual service should include compression testing, injector inspection, winding tests, and full ATS functional testing by a qualified generator technician.
The 5 most common generator maintenance mistakes
Running at low load
Running diesel generators below 30% load causes wet stacking — unburned fuel accumulates in the exhaust system, leading to carbon buildup, injector fouling, and premature engine wear. Always test under meaningful load.
Neglecting the fuel tank
Diesel degrades over 12 months and is susceptible to water contamination and microbial growth (diesel bug). Annual fuel sampling and periodic fuel polishing are essential for stored reserves.
Skipping the ATS test
The automatic transfer switch is the single most critical component in a backup power system — and the most commonly skipped in maintenance programs. Test it fully every month.
Ignoring battery age
Generator batteries should be replaced every 2–3 years regardless of how healthy they appear. A battery can show 12.6V at rest but fail completely under the cranking load at -10°F.
No fuel delivery plan
The most sophisticated maintenance program fails if there's no fuel when you need it. Establish a scheduled on-site fuel delivery relationship with a 24/7 provider before an emergency arises.
Generator maintenance task quick reference
Use this summary table for quick scheduling reference or to build your own maintenance calendar.
| Task | Interval | Who performs | NFPA 110 |
|---|---|---|---|
| Fuel level check | Daily | Operator | Required |
| Oil & coolant level check | Daily | Operator | Required |
| Alarm / fault code check | Daily | Operator | Required |
| Test run (unloaded) | Weekly | Operator | Recommended |
| ATS functional test | Weekly | Operator | Required |
| Air filter inspection | Weekly | Operator | Recommended |
| Load bank test (30% min) | Monthly | Technician | Required |
| Fuel filter drain/inspect | Monthly | Technician | Recommended |
| Coolant concentration check | Monthly | Technician | Recommended |
| Oil & filter change | 6 months / 250 hrs | Technician | Required |
| Battery load test | 6 months | Technician | Required |
| Fuel & oil filter replacement | 6 months / 250 hrs | Technician | Required |
| Compression test | Annual | Technician | Recommended |
| Injector inspection/test | Annual | Technician | Recommended |
| Battery replacement | 2–3 years | Technician | Required |
| Winding insulation test | Annual | Technician | Recommended |
| Full ATS inspection & test | Annual | Technician | Required |
| Fuel quality lab sample | Annual | Lab | Recommended |
For critical facilities operating under NFPA 110 Level 1 requirements (hospitals, emergency services, data centers), all maintenance must be performed by a qualified technician and fully documented. Level 2 facilities have slightly more flexibility but still require documented monthly load testing and annual full service.
If your generator supports a critical operation and you don't yet have a scheduled fuel delivery plan in place, that's the most important single action you can take today. Emergency fuel delivery is available 24/7 from FuelGo, but pre-arranged scheduled delivery is always faster, cheaper, and more reliable than emergency dispatch. Call us at 800-720-4546 to set up a delivery schedule for your site.