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Process Design

Tank Freeboard Calculator

Freeboard is the headroom left above the operating liquid level in a tank, kept clear for surge, foam, wave action, and overflow protection — so tanks are almost never run completely full. This calculator estimates the total tank volume, the working (liquid) volume, the freeboard height, the empty ullage volume, and the fill percentage for a rectangular or vertical cylindrical tank. You can work either way: enter the liquid depth to find the freeboard, or enter the freeboard to find the liquid depth. It is a preliminary geometric estimate only — it does not set safety, overflow, or bunding requirements and is not a mechanical design.

TypeInteractive engineering calculator

Calculator

m
m
m
m
Result
Total tank volume24 m³
Working / liquid volume18 m³
Liquid depth1.5 m
Freeboard height0.5 m
Freeboard / ullage volume6 m³
Fill percentage75 %

Preliminary geometric estimate only. Does not set safety, overflow, or bunding requirements and is not a mechanical design. Ignores wall thickness, internals, nozzles, foam, wave action, vortexing, and sloped floors. Use verified drawings, operating philosophy, and qualified engineering review for design work.

Formulas

Rectangular total volume
V_total = L × W × H_total
Cylinder total volume
V_total = (π·D²/4) × H_total
Working volume
V_liquid = A × H_liquid
Freeboard
freeboard = H_total − H_liquid
Ullage volume
V_ullage = V_total − V_liquid
Fill fraction
fill = H_liquid / H_total

Diagram

Freeboard = H_total − H_liquidfreeboard(ullage)workingvolumeH_totalliquid levelfreeboard = H_total − H_liquid

Worked example

A rectangular tank is 4.0 m long, 3.0 m wide, and 2.0 m high. The liquid depth is 1.5 m. What are the working volume, freeboard, and ullage?

  1. 01V_total = L × W × H_total = 4.0 × 3.0 × 2.0 = 24.0 m³
  2. 02V_liquid = L × W × H_liquid = 4.0 × 3.0 × 1.5 = 18.0 m³
  3. 03freeboard = H_total − H_liquid = 2.0 − 1.5 = 0.5 m
  4. 04V_ullage = V_total − V_liquid = 24.0 − 18.0 = 6.0 m³
  5. 05fill = H_liquid / H_total = 1.5 / 2.0 = 75.0%
Result

Total volume = 24.0 m³; working volume = 18.0 m³; freeboard = 0.5 m; ullage = 6.0 m³; fill = 75.0%.

FAQ

What is the difference between freeboard and ullage?
Freeboard is the vertical headroom left above the liquid level — a height, set for surge, foam, and overflow protection. Ullage is the empty volume above the liquid at a given level. Freeboard is usually treated as a design allowance; ullage is simply the unfilled space at the current depth.
Why are tanks not run completely full?
Freeboard absorbs surge from flow imbalance, foam, wave action and splashing from agitation, and gives margin before overflow. Running a tank to the brim removes that margin and risks spills, so an operating freeboard is kept above the normal high level.
Can I enter the freeboard instead of the liquid depth?
Yes. Switch the mode to calculate the liquid depth from a freeboard you specify; the calculator subtracts the freeboard from the total height to get the working depth, then computes the working and ullage volumes.
Does this set the required freeboard for my tank?
No. This is a geometric estimate. The required freeboard comes from the operating philosophy, surge cases, foam behaviour, overflow and bunding requirements, and project standards — not from geometry alone.

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