About Plumbing & Drainage Sizing Calculator
The plumbing and drainage sizing calculator converts fixtures into drainage fixture units (DFU) and water supply fixture units (WSFU), estimates design flow, and selects pipe diameters per IPC, EN 12056, and AS/NZS 3500. It is used by plumbing engineers to size soil stacks, drains, and supply mains and to verify self-cleansing velocity.
Add each fixture type and quantity, choose the standard and pipe material, and set the run length. The tool returns the total fixture units, the design flow rate, the recommended pipe diameter, the flow velocity, the minimum gradient, and a self-cleansing check, all updating in real time.
How It Works
- Sum the drainage fixture units (DFU) and water supply fixture units (WSFU) by multiplying each fixture value by its quantity.
- Convert fixture units to a design flow rate using the Hunter curve (or the EN 12056 root-sum-of-squares method for drainage).
- Select the pipe diameter from the standard fixture-unit capacity table and read the minimum gradient for that size.
- Compute the flow velocity v = Q / A and confirm it lies between 0.6 and 3.0 m/s for self-cleansing.
Worked Example
Two WCs (3 DFU each), three lavatories (1 DFU each), and one shower (2 DFU) give a total of 2*3 + 3*1 + 1*2 = 11 DFU. The Hunter drainage flow is Q = 0.0084 * 11^0.613 = 0.052 L/s, and 11 DFU falls within the 75 mm pipe capacity, which carries a minimum gradient of 1:40.
Formulas
- Total drainage fixture units
DFU = sum(dfu_i * quantity_i)- Hunter drainage flow
Q = 0.0084 * DFU^0.613- Hunter supply flow
Q = 0.013 * WSFU^0.555- EN 12056 drainage flow
Q = K * sqrt(sum(DU^2))- Flow velocity
v = Q / (pi * (D/2)^2)
Standards & References
- IPC
- EN 12056
- AS/NZS 3500
Frequently Asked Questions
What is a fixture unit?
A fixture unit is a dimensionless measure of the probable load a fixture places on a system. Drainage fixture units (DFU) size drains and water supply fixture units (WSFU) size supply pipes; a WC contributes far more than a lavatory.
How is the design flow calculated?
Fixture units are converted to a probable flow with the Hunter curve, Q = 0.0084 * DFU^0.613 for drainage and Q = 0.013 * WSFU^0.555 for supply, or with the EN 12056 root-sum-of-squares method Q = K * sqrt(sum(DU^2)) for drainage.
What is the self-cleansing velocity?
Drains must run fast enough to carry solids but not so fast as to erode the pipe. The tool checks that the flow velocity stays between 0.6 and 3.0 m/s and flags a fail outside that band.
Which standards does it support?
It supports the IPC and AS/NZS 3500 fixture-unit methods using the Hunter curve, and the EN 12056 design-unit method using the root-sum-of-squares of fixture discharge units.