How we calculate: plumbing
Every formula and default figure our plumbing calculators use. Each figure can be changed in its calculator to match your product. See how we calculate for the method behind all of them.
Hot water cylinder size calculator
Open the hot water cylinder size calculator
- Storage = people × litres per person + extra per bathroom after the first, then the next stock size up
- Energy (kWh) = litres × 4.186 × (storage temperature − cold feed) ÷ 3600
- Reheat time (h) = energy ÷ heat input (kW): the coil output, not the boiler's or heat pump's full output
Unvented cylinders must be fitted by a competent person (Building Regulations G3) and need an expansion vessel or air bubble.
Any gas boiler work, including connecting a cylinder coil, is for a Gas Safe registered engineer.
Heat pump cylinders are usually larger with a bigger coil: follow the heat pump maker's and MCS guidance.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Stored hot water per person: Showers | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB4: litres of stored hot water per person, mostly showers. Researcher to source to MCS MIS 3005-D or a cylinder maker. |
| Stored hot water per person: Baths | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB4: litres of stored hot water per person, mostly baths. Researcher to source to MCS MIS 3005-D or a cylinder maker. |
| Extra storage per bathroom after the first | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB4: extra storage per bathroom after the first. Researcher to source to a cylinder maker's sizing guide. |
| Storage temperature | 60 °C | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB4 (HW1 test value): storage temperature. Researcher to cite (HSE L8 / cylinder maker). |
| Cold feed temperature | 10 °C | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB4 (HW1 test value): cold feed temperature. Researcher to cite. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| Hot water cylinder stock sizes TODO: Researcher to add the maker or standard URL | Cylinder makers' ranges (Researcher to fill from at least two UK makers) | UK-wide | 0/8 values entered Not yet checked against the source |
Water flow rate calculator
Open the water flow rate calculator
- Flow (L/min) = litres collected ÷ seconds × 60
- L/s = L/min ÷ 60; m³/h = L/s × 3.6
- Pressure drop = static pressure − dynamic (running) pressure
Test at the tap nearest the stopcock (usually the kitchen cold tap) with nothing else running, and repeat it two or three times.
Combi boilers, unvented cylinders and power showers each list a minimum flow and pressure: compare with the maker's figure, not a general rule.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Expansion vessel size calculator
Open the expansion vessel size calculator
- Expansion volume = system volume × e (the fraction the water expands by from cold to the maximum temperature)
- Acceptance factor = 1 − (pre-charge + 1) ÷ (maximum pressure + 1), using absolute pressures (gauge + 1 bar)
- Vessel size = expansion volume ÷ acceptance factor, then the next stock size up
For a heating system with no measured volume, ask the installer: radiator and boiler makers give water contents.
Indicative only: the installer sizes the vessel to the maker's instructions and sets the pre-charge.
This calculator has no built-in defaults: every figure comes from you or your product's data.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Expansion of water from cold fill to the stated temperature TODO: Researcher to add the maker or standard URL | BS 7074-1 / water property tables (Researcher to fill and cite) | UK-wide | 0/9 values entered Not yet checked against the source |
| Expansion vessel stock sizes TODO: Researcher to add the maker or standard URL | Expansion vessel makers' ranges (Researcher to fill from at least two UK makers) | UK-wide | 0/8 values entered Not yet checked against the source |
Pipe size and velocity calculator
Open the pipe size and velocity calculator
- Flow Q (m³/s) = L/min ÷ 60 ÷ 1000; bore area A = π × d² ÷ 4 (d = internal diameter in m)
- Velocity v = Q ÷ A
- Head loss (Hazen–Williams) h = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.87), with L the run plus the fittings allowance
- Pressure loss (bar) = head loss (m) × 0.0981
Water pipes only. Gas pipework is sized by a Gas Safe registered engineer and isn't covered here.
Head loss is shown only when you enter the Hazen–Williams C for your pipe.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Velocity limit | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB2: velocity limit for choosing a pipe size. Researcher to source to BS EN 806-3 / CIBSE guidance. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| Pipe internal diameters by material and outside diameter TODO: Researcher to add the maker or standard URL | BS EN 1057 (copper); makers' data for PEX and MDPE (Researcher to fill) | UK-wide | 0/9 values entered Not yet checked against the source |
Water pressure from height calculator
Open the water pressure from height calculator
- Pressure (bar) = head (m) × 0.0981
- Head (m) = pressure (bar) ÷ 0.0981
- 1 bar = 100 kPa
This is the static pressure with nothing running; the pressure at the outlet drops when water flows.
Whether a gravity shower needs a pump depends on the shower: compare with the maker's minimum head or pressure.
This calculator has no built-in defaults: every figure comes from you or your product's data.