oil_barrel Tank Volume Calculator (Partial Fill) star

How It's Calculated

The three tank shapes are not equally hard. A vertical cylindrical tank is simplest: V = π·r²·h, and because the cross-section never changes with height, fill level and volume rise together in a straight line — 50% height is always exactly 50% volume. A rectangular tank is simpler still: V = length × width × height, again perfectly linear.

A horizontal cylindrical tank is the genuinely hard case, and the one people usually reach for a calculator over. Lying on its side, liquid at any dip height h no longer fills a simple rectangle — it fills a circular segment, the curved slice below a horizontal chord. That area has no simple shortcut; it comes from integrating the circle's cross-section: A = r²·acos((r−h)/r) − (r−h)·√(2rh−h²). Multiply by the tank's length for partial volume. This is exactly the "I measured 40 cm with a dipstick, how many liters?" question a naive πr²h guess gets wrong, since segment area does not grow at a constant rate as h increases.

Worked example: a horizontal tank 3 m long with a 1.2 m radius (2.4 m diameter), dipped to h = 0.6 m — half the radius, not half the diameter. Half the diameter (h = r = 1.2 m) gives exactly 50% by symmetry. But at h = 0.6 m the segment area is well under half of πr², because the segment near the bottom is proportionally thinner for a given height than near the middle — a dip at 25% of the diameter holds only around 20% of total volume, not 25%.

What You Should Know

Total capacity is the same regardless of orientation — π·r²×length — but how it maps to a dipstick reading differs completely between vertical and horizontal installation. This calculator reports current liters, total capacity, fill percentage and remaining headspace for both, using the correct formula for each.

• Flat ends assumed: real tanks often have dished or conical heads, adding a few percent of extra volume near the very top and bottom compared to a flat-ended tank of the same length.
• Level installation assumed: a horizontal tank tilted even slightly changes the true liquid surface geometry, and both the dipstick reading and calculated volume drift off.
• Custody transfer / fuel sales: when a reading determines an invoice, use the manufacturer's calibrated strapping chart rather than a generic formula — it accounts for that tank's actual heads, fittings and any out-of-round distortion.

As a rule of thumb, a level tank with flat ends and a good dipstick reading gets within 1-2% of the true volume — enough for routine monitoring, refill planning and leak-rate estimates, even if not for a legal handover.