NFPA 22 Break Tank & Tank Refill Sizer

Tank Sizing

Flow Required (gpm) Duration (min)

Total Gallons Required: 0.000 gal

Desired Tank Size (gal)

Reliable Tank Fill Requirement

0.000 gpm

Enter tank sizing values to compare the required refill rate with the flow calculator.

The tank fill calculation follows the formula: 1.1 × (Total Gallons Required − Desired Tank Size) ÷ Duration.

Flow Calculator

Q = 29.84 × C × d² × √P

Discharge Coefficient, C Hole / Pipe Inside Diameter, d (in.) Pressure at Hole, P (psi)

Calculated Flow

0.000 gpm

This calculated flow can be compared with the reliable tank fill requirement.

Discharge Coefficient Note: The default Discharge Coefficient is C = 0.65. This represents a clean-edged hole in the side of a tank. In high importance situations, a coefficient of 0.50 may be more accurate and would be more conservative.

Tank Refill Check

Reliable Tank Fill Requirement

0.000 gpm

Flow Calculator Result

0.000 gpm

Results update automatically as values are changed.

Notes

GPM Flow Rate Through a Hole

To calculate the Gallons Per Minute (GPM) flow rate through a hole, you primarily need the hole's diameter and the water pressure. The most common formula used for this is the Orifice Flow Equation.

The Orifice Flow Formula

The standard equation to estimate GPM through a circular hole is:

Q = 29.84 × C × d² × √P

Q: Flow rate in GPM.
C: Discharge coefficient (typically 0.60 to 0.65 for a sharp-edged hole).
d: Diameter of the hole in inches.
P: Pressure at the hole in PSI.

Where the 29.84 Comes From

In the orifice flow equation, 29.84 is a mathematical constant used to convert standard physical units into Gallons Per Minute (GPM).

It consolidates several different conversion factors into a single number so you can use the most common field measurements (inches and PSI) directly.

Where does it come from?

The original physics formula for flow is Q = A · v, where velocity (v) is derived from Bernoulli's principle as √2gh. To get 29.84, you take that base formula and apply the following conversions:

Diameter to Area: Converts the hole diameter in inches into square feet (π · d² / 4 · 144).
Pressure to Head: Converts PSI (pounds per square inch) into feet of "head" (water height) using the density of water.
Gravity (g): Incorporates the constant for Earth's gravity (32.2 ft/s²).
Volume & Time: Converts cubic feet per second into gallons per minute.

When to Use It

This specific constant is the standard used in NFPA 291 for fire hydrant flow testing and irrigation engineering to quickly estimate discharge rates from an open pipe or orifice.