Tank dip chart for cylindrical tanks

Contents

1. What a dip chart is
2. How to make a dip chart
3. Example chart: horizontal tank 48 in × 72 in
4. Metric example: tank 1.2 m × 2.5 m, every 10 cm
5. How to read the dipstick
6. Vertical tanks: a linear chart
7. Strapping: checking a chart against the real tank
8. Common mistakes
9. FAQ

The calculator above takes the inside diameter, the length and a depth, and opens on its Chart tab: it lists the volume at every step of depth (a tenth or a twentieth of the height, or every unit), in gallons and liters, then print the chart and keep it by the tank.

What a dip chart is

A dip chart, also called a gauge chart, a tank chart or a strapping chart, is a two-column table: depth on one side, volume on the other. You push a stick to the bottom of the tank, read the wet line, and look up the volume. It is the simplest way to know how much fuel, water or oil is left without a flow meter or a level sensor.

For a standing tank, the chart is almost unnecessary: the volume grows in a straight line with the depth. For a tank lying on its side, the relation is curved, and a chart is the practical way to avoid working out the horizontal cylinder volume formula every time you dip the tank.

How to make a dip chart

1. Measure the inside diameterMeasure the outside, then subtract twice the wall thickness. If you can only reach around the tank, divide the circumference by π.
2. Measure the lengthMeasure the straight shell from end to end. Leave out any domed ends; they need a separate allowance.
3. Choose the stepEvery inch for a small tank, every 2 in for most home tanks, every centimeter or every 2 cm in metric. Finer steps make reading easier; they do not make the tank measurements more accurate.
4. Compute each rowFor a horizontal tank, V = L × [r² acos((r − f) ÷ r) − (r − f) √(2rf − f²)] at each depth f. For a vertical tank, V = π × r² × f.
5. Convert and printDivide cubic inches by 231 for US gallons, or cubic centimeters by 1,000 for liters. Print the chart with the tank's dimensions written at the top.

The tool above follows these steps once you enter the diameter and the length. The page on cylindrical tank volume explains how to take the measurements on tanks of different shapes.

Example chart: horizontal tank 48 in × 72 in

This chart is computed for a flat-ended horizontal cylinder with an inside diameter of 48 in and a length of 72 in. Full, it holds 130,288.1 in³ = 564.0 US gallons (2,135.0 L). The depth is measured from the bottom of the tank.

Look at the step between rows. From 0 to 2 in, the tank gains 8.0 gallons. From 22 to 24 in, around the middle, it gains 29.9 gallons. From 46 to 48 in, at the top, it gains 8.0 gallons again. The chart is symmetric: the volume at a depth f plus the volume at 48 − f always makes 564.0 gallons.

This chart only fits a tank with these exact inside dimensions. Do not use it for another tank of similar capacity: a tank that is narrower and longer gives different volumes at the same depth. Build a new chart for each tank.

Metric example: tank 1.2 m × 2.5 m, every 10 cm

A horizontal tank 1.2 m across the inside and 2.5 m long holds π × 0.6² × 2.5 = 2.8274 m³, or 2,827.4 liters (746.9 US gallons).

How to read the dipstick

• Dip at the middle. Use an opening near the center of the top, so the stick reaches the lowest point of the circle and stands vertical. A stick that leans reads too deep.
• Touch the bottom, then pull out straight. Read the wet line at once, before it runs or dries. For a clear liquid, a light coat of chalk on the stick makes the line easier to see.
• Interpolate between rows. At 15 in in the chart above, halfway between 136.8 gallons (14 in) and 164.6 gallons (16 in) gives 150.7 gallons; the exact segment calculation gives 150.6. Over a 2 in step, straight-line reading between two rows is close enough for most uses.
• Check the tank is level. On a tilted tank, the depth at the dip point is not the depth along the whole tank, and the chart reads high or low.

Mark the stick itself. Using the chart, cut a notch or draw a line at the depth for each 50 or 100 gallons. Next time, you read the volume straight off the stick.

Vertical tanks: a linear chart

A standing cylinder has the same cross-section at every height, so each inch of depth adds the same volume. For a vertical tank 48 in across, each inch holds π × 24² ÷ 231 = 7.834 US gallons. At 30 in of depth it holds 30 × 7.834 = 235.0 gallons, and at 72 in it holds 564.0 gallons. In metric, a vertical tank 1.2 m across gains 11.31 L per centimeter. The chart reduces to one number per inch or per centimeter; the per-inch values for common diameters are in the table on the tank volume calculator page.

Strapping: checking a chart against the real tank

A calculated chart assumes a perfect cylinder with flat ends. Real tanks have rounded heads, small dents, internal fittings, and a sump or outlet that may hold liquid the stick does not reach. To check or correct a chart, fill the tank in measured amounts (with a meter or a container of known size) and record the depth after each addition. Where your readings differ from the calculation, trust the readings. The outlet line also holds liquid that a dipstick cannot see; the pipe volume calculator gives the gallons per foot of a line from its inside diameter. For tanks with hemispherical ends, the added volume is covered on the cylinder with hemisphere volume page.

Common mistakes

• Using outside dimensions. The chart must use the inside diameter, or every row comes out too high.
• Using a linear chart for a horizontal tank. At a quarter of the depth, a horizontal cylinder holds 19.6 % of its volume, not 25 %.
• Mixing gallon types. A US gallon is 3.785411784 L, a UK gallon 4.54609 L. See cylinder volume in gallons for the conversions.
• Counting the length with the domes. Measure only the straight shell for L, then allow for the heads separately.

To check the full capacity before building a chart, the basic cylinder volume calculator gives π × r² × L in any unit.

FAQ

What is a tank dip chart?

A table that converts the depth of liquid, read on a dipstick, into a volume. Each row gives a depth and the gallons or liters held at that depth. It is also called a gauge chart, a tank chart or a strapping chart.

How do I make a dip chart for my tank?

Measure the inside diameter and the length of the tank, choose a depth step (every inch, every 2 in or every centimeter), and compute the volume at each depth. For a horizontal tank use the circular segment formula; for a vertical tank multiply the base area by the depth. The calculator's Chart tab does it and prints the result.

How do I read a dipstick between two lines of the chart?

Interpolate: take the two rows around your reading and split the difference in proportion. At 15 in in a 48 in × 72 in tank, halfway between 136.8 and 164.6 gallons gives 150.7 gallons; the exact value is 150.6.

Why are the steps uneven on a horizontal tank chart?

Because the liquid surface is narrow near the bottom and top and widest at the middle. In a 48 in × 72 in tank, the bottom inch holds about 2.9 gallons and an inch at mid-height about 15 gallons.

Can I use one chart for two tanks of the same capacity?

Only if they have the same inside diameter and length. Two tanks with the same capacity but different proportions give different volumes at the same depth.

Related

• Horizontal cylinderHorizontal tank volume calculator for a partly filled cylinder: segment formula, depth to volume table, and a worked 48 in × 96 in example in gallons.
• Cylinder tank volumeTank volume calculator for vertical and horizontal cylinder tanks, full or partly filled. Get the capacity in gallons and liters, with worked examples.
• Cylinder volume in gallonsFind the volume of a cylinder in gallons: π × r² × h in inches ÷ 231, or in feet × 7.48052. US and imperial gallons, worked examples and a capacity chart.

Updated October 12, 2026