Diesel & Fuel Tank Calculator — Gallons, Weight & Dip Chart

Why Diesel Weighs Less Than Water — Specific Gravity, LB/gal & Lb to KG

If you have ever lifted a full 5-gallon jug of diesel and a 5-gallon jug of water back-to-back, you already know the headline: diesel feels lighter. The reason is density. Water has a specific gravity of 1.000, which is the reference point every other liquid is measured against. No. 2 on-highway diesel has a specific gravity of roughly 0.84, meaning it is 84% as dense as water. Do the math: water is 8.34 lb per gallon, so diesel is 0.84 × 8.34 = 7.0 lb per gallon. That 1.34 lb difference per gallon sounds small until you multiply it across a 1,000-gallon bulk tank — that tank holds 7,000 lb of diesel but would hold 8,340 lb of water. The difference is more than the curb weight of a full-size pickup.

This density gap is the single most important number in every diesel tank calculation because it feeds directly into three of the biggest downstream questions: how much weight you can add to a truck before hitting GVWR, how many gallons you have left after a dip, and whether your generator day tank will hold enough fuel for a 24-hour outage. The calculator on this page exposes the SG as a tunable input so you can get the right weight for winter-blend diesel (SG down to 0.82), biodiesel B20 (SG around 0.85), or even kerosene (SG ≈ 0.81).

Standard Diesel Tank Sizes — Truck Saddle, Generator Day Tanks & Bulk Storage

When you pull out a tape to measure a fuel tank, it almost always falls into one of three families: truck saddle tanks that hang off the side rails of pickup and semi-trucks, generator day tanks that gravity-feed a genset, andbulk storage tanks that sit permanently on a yard pad or job site. Each has a typical capacity range and each presents a different challenge for dip reading and delivery math. The table below gives the common sizes you will encounter in the field.

Dip Chart Math for Horizontal Oval & Cylindrical Tanks

The most common mistake in a diesel tank measurement is treating a horizontal oval or cylindrical tank as if its dip reading were proportional to its height. On a straight rectangular tank, if you dip at 50% of height, you have 50% of the fuel. On a horizontal cylinder, a dip at 50% of diameter gives you 100% of the fuel — because the cylinder's full round cross-section is horizontal. On a horizontal oval truck saddle tank, the relationship is in between: the narrow top and bottom ends hold less fuel per inch than the fat middle, so a 10% dip from the bottom is only about 2% of full capacity, and a dip 90% from the bottom is only about 98% full.

That non-linear relationship is why a home-made stick with inches marked on it will never read correctly on a truck saddle tank. What you need is a dip chart. This page's calculator generates a value for whatever dip height you enter using a proper circular-segment formula for horizontal cylinders and a proportional-area approximation for oval tanks — good enough to spot-check the official dip chart from the tank manufacturer or calibrate your own stick.

Delivery Math — How Much Fuel to Top Up & How Much It Weighs

The single most common question field techs and fleet managers ask about a diesel tank is: how much fuel do I need to bring out on my next delivery? The formula is simple, but doing it wrong lands you at the end of the run with an empty generator or a truck that is overweight on the scale.

Step one is to take a valid dip. Do not guess. Step two is to read the current gallons from your dip chart or from this calculator's partial-fill number. Step three is to subtract that current reading from the tank's usable capacity (not its label capacity — use the manufacturer's "usable" number or 95% of the geometric full). The result is the number of gallons the delivery driver should add. Multiply by 7.0 lb/gal (or your actual SG × 8.34) and you get how many pounds of load those gallons add to the truck — critical for legal weight-bridge compliance.

Road Weight Limits & the Real Cost of a Full Tank

US federal law caps the gross vehicle weight rating (GVWR) of a standard five-axle tractor-trailer at 80,000 lb. Class 6 and 7 medium-duty trucks run 19,500 to 26,000 lb GVWR. A typical Class 8 semi tractor itself — the chassis, engine, cab and two saddle tanks — weighs about 18,000 lb empty, leaving 62,000 lb of payload capacity for the trailer and its cargo. A full 100-gallon saddle tank adds 700 lb. Two of them (the usual configuration) add 1,400 lb. Fill both tanks to the brim and you are already carrying nearly 1.5 short tons of pure fuel before you have touched the trailer.

For pickup trucks used for hauling, the math is tighter. A half-ton (1500 series) pickup has a payload capacity of roughly 1,800–2,500 lb. A 50-gallon in-bed auxiliary tank full of diesel adds 350 lb, a rack and tool box add another 200 lb, and a driver plus passenger add 400 lb. All of a sudden you are down to less than 1,000 lb of real hauling capacity — less than half the sticker "half-ton" number suggests. The reason fleet managers care so much about accurate diesel tank weight calculation is that an overweight citation at a scale house runs $300–$10,000 depending on the state and how far over you are, and the DOT will shut you down for 10+ hours if you are more than 10% over.

Ullage — Why Your Tank Must Never Be Filled to the Top

Diesel fuel is a liquid, and liquids expand when they warm up. The coefficient of thermal expansion for diesel is roughly 0.0007 per °F — meaning a gallon expands by about 0.07% for every degree of temperature rise. Fill a 100-gallon tank in the morning at 40 °F to 100 gallons exactly, drive it all day in 90 °F heat, and the tank now wants to hold 100 × 1.0007⁵⁰ = 100 × 1.0356 = 103.6 gallons. The extra 3.6 gallons has nowhere to go. Without ullage headspace, you get blow-back through the vent (illegal on many roads), fuel weeping onto the pavement, or a damaged cap seal.

The industry standard is to leave 5–10% ullage in every fuel tank. On a 100-gallon tank that is 5–10 gallons of headspace. The calculator on this page reports ullage automatically based on the fill height you entered — you can dial the dip down from "full" until the ullage number sits in that safe 5–10% band. This is also why tank manufacturers quote a "usable" capacity on top of the geometric capacity — the usable number already subtracts the required headspace.

Frequently Asked Questions