Get pipe sizing wrong and you’ll know about it quickly. Low pressure at the top-floor shower. Noisy pipes banging behind the walls. A pump running constantly because it’s fighting a system that was never designed properly. These are not random faults — they are almost always the result of incorrect pipe sizing at the design stage.
Pipe sizing for plumbing systems is one of the most practical skills an MEP engineer or facilities manager can have. Whether you’re reviewing a contractor’s design, planning a small extension to an existing system, or just trying to understand why a building’s hot water pressure is terrible — this guide gives you the knowledge to work it out.
We’ll cover the core principles, the calculation method, the pipe sizing tables you need, and the common mistakes that cause problems on site. No complicated theory — just practical knowledge you can apply straight away.
Figure 1: Plumbing Pipe Sizing — Flow Hierarchy and Velocity Targets
| PLUMBING PIPE SIZING — FLOW HIERARCHY & VELOCITY TARGETS | |
| 🏗️ Water Supply Source | Utility main / borehole / rooftop tank. Largest diameter pipe in system. |
| ↓ Main Riser / Service Pipe | Largest diameter — lowest velocity. Carries full building load. Size for total LU. |
| ↓ Zone / Floor Main 50–100mm | Velocity: 0.5–1.5 m/s. Serves an entire floor or zone. Never undersize this. |
| ↓ Branch Pipe 32–50mm | Velocity: 1.0–2.0 m/s. Feeds groups of fixtures on a corridor or zone. |
| ↓ Sub-Branch 20–32mm | Velocity: 1.5–2.5 m/s. Feeds 2–4 fixtures directly. |
| ↓ Fixture Supply 15–20mm | Velocity: ≤3.0 m/s. Direct connection to WC, basin, shower, or tap. |
| ⚠️ Key Rule: As pipe gets smaller → velocity increases → friction loss increases → size carefully to avoid noise, erosion, and pressure drop. | |
| Pipe Diameter Formula: D = √ ( 4Q / π·V ) × 1000 | Q = Flow Rate (m³/s) | V = Velocity (m/s) | |
What Is Pipe Sizing and Why Does It Matter?
Pipe sizing means selecting the correct internal diameter for every pipe in a plumbing system — from the main service entry all the way to the individual fixture at the end of the line. Get it right and the system works quietly and efficiently. Get it too small and you get high velocity, noise, erosion, and pressure drop. Get it too large and you waste money on pipe and fittings, and end up with stagnant water in oversized sections.
Three things control pipe sizing: flow rate (how much water needs to pass through), velocity (how fast it travels), and pressure (what’s available and what’s needed at the outlet). Every sizing decision is a balance between these three factors.
In a commercial building, you might have dozens of fixtures all connected to a single main supply. The challenge is making sure every fixture gets the right flow and pressure, even when multiple outlets are running at the same time.
| 💡 Quick Rule of Thumb: For cold water pipes in commercial buildings, target a water velocity of 1.0–2.0 m/s in branch pipes and no more than 3.0 m/s at fixture connections. Exceeding this causes noise, erosion, and pipe movement. |
Key Terms You Need to Know Before You Start
Before we get into the calculation method, here are the terms you’ll see in every pipe sizing table and standard:
• Flow Rate (Q) — the volume of water flowing through the pipe per second, measured in litres per second (L/s) or cubic metres per second (m³/s). This comes from the number and type of fixtures connected.
• Velocity (V) — how fast the water moves through the pipe, in metres per second (m/s). Too fast = noise and erosion. Too slow = stagnation and Legionella risk.
• Pipe Diameter (D) — the internal bore of the pipe in millimetres. This is what you’re calculating.
• Friction Loss — the pressure drop caused by water moving through the pipe. Longer pipes, smaller diameters, and higher velocities all increase friction loss.
• Loading Units (LU) — a standardised way of expressing the demand of each fixture type. A WC might be 5 LU, a washbasin 1.5 LU. You add these up to get the total flow demand.
• Simultaneous Demand — not all fixtures run at the same time. Pipe sizing accounts for the realistic probability that only a percentage of fixtures will operate at once.
How to Size Pipes for a Plumbing System — Step by Step
This is the Equal Pressure / Loading Unit method used in most commercial plumbing design, based on BS EN 806 and CIBSE Guide G standards. It works for domestic, commercial, and institutional buildings worldwide.
Step 1 — List All Your Fixtures
Start by listing every plumbing fixture in the building or zone you’re sizing for. For each fixture, note the type and the Loading Unit (LU) value from the standard table. Common values are:
| Fixture Type | Cold Water LU | Hot Water LU | Min. Flow Rate (L/s) |
| WC — cistern flush | 5 | — | 0.10 |
| WC — flush valve | 10 | — | 1.50 |
| Washbasin (tap) | 1.5 | 1.5 | 0.10 |
| Shower | 3 | 3 | 0.15 |
| Kitchen sink | 3 | 2 | 0.20 |
| Bath | 10 | 10 | 0.30 |
| Urinal — cistern | 0.3 | — | 0.15 (per bowl) |
| Dishwasher (commercial) | 5 | 5 | 0.20 |
| Washing machine (domestic) | 3 | 3 | 0.20 |
Source: BS EN 806-3 and CIBSE Guide G — Public Health Engineering. Use your local standard if a different code applies on your project.
Step 2 — Calculate Total Loading Units
Add up the Loading Units for all fixtures that share the same pipe section. For example, if a branch pipe serves 4 washbasins and 2 showers on a floor:
• 4 washbasins × 1.5 LU = 6 LU
• 2 showers × 3 LU = 6 LU
• Total = 12 LU for that branch pipe
Step 3 — Convert Loading Units to Flow Rate
Loading Units don’t directly equal flow rate. The conversion accounts for simultaneous demand — the realistic probability that not all fixtures run at the same time. Use the chart from BS EN 806-3 or the simplified formula:
| 📐 Formula: Q (L/s) = 0.682 × LU^0.45 − 0.14 (for total LU between 2 and 5000) Example: 12 LU → Q = 0.682 × 12^0.45 − 0.14 ≈ 0.682 × 3.12 − 0.14 ≈ 1.99 L/s |
Step 4 — Select the Pipe Diameter
Now use the flow rate and your target velocity to calculate the minimum pipe diameter:
| 📐 Formula: D (mm) = √ (4Q / π·V) × 1000 Where Q = flow rate in m³/s and V = target velocity in m/s Example: Q = 1.99 L/s = 0.00199 m³/s, V = 1.5 m/s D = √ (4 × 0.00199 / π × 1.5) × 1000 = √ (0.001688) × 1000 ≈ 41 mm → select 50 mm pipe |
Always round up to the next standard pipe size — never round down. Standard pipe sizes are: 15 mm, 20 mm, 25 mm, 32 mm, 40 mm, 50 mm, 65 mm, 80 mm, 100 mm.
Step 5 — Check Velocity and Pressure Loss
After selecting the pipe size, verify the actual velocity at that diameter and then calculate the friction loss over the pipe length. For long pipe runs, you may need to upsize to keep pressure at the furthest fixture within acceptable limits.
• Target velocity check: V = Q / A, where A = π × (D/2)²
• Pressure loss check: Use manufacturer friction loss charts or the Darcy-Weisbach equation. A typical acceptable friction loss for commercial plumbing is 0.3–0.5 kPa/m
• Minimum residual pressure at fixture: WC cistern = 50 kPa, Shower = 100 kPa, Flush valve WC = 200 kPa
Pipe Sizing Quick Reference Table
Use this table for fast reference when sizing common pipe sections in commercial buildings. Based on target velocity of 1.5 m/s for branch pipes:
| Pipe Size (mm) | Internal Bore (mm) | Flow Rate at 1.5 m/s (L/s) | Typical Application |
| 15 mm | 13.6 | 0.22 | Individual fixture supply — tap, shower |
| 20 mm | 18.6 | 0.41 | Short fixture branches — 2–3 fixtures |
| 25 mm | 22.8 | 0.61 | Small branch — up to 5 LU |
| 32 mm | 29.0 | 1.00 | Floor branch — 5–15 LU |
| 40 mm | 35.2 | 1.46 | Riser or large branch — 15–40 LU |
| 50 mm | 44.3 | 2.31 | Zone main — 40–100 LU |
| 65 mm | 59.0 | 4.10 | Building main — 100–250 LU |
| 80 mm | 72.9 | 6.26 | Large building main — 250–600 LU |
| 100 mm | 94.0 | 10.40 | Main service entry — 600+ LU |
| Use the free Duct Sizing Calculator to run velocity and pressure checks quickly. Free Duct Sizing Calculator → |
Pipe Material Selection — What Goes Where
The material you choose affects diameter, friction factor, and longevity. Different materials are common in different regions and project types:
| Material | Common Use | Advantage | Watch Out For |
| Copper (BS EN 1057) | Hot and cold water, internal | Durable, biostatic, well understood | High cost; dezincification risk in aggressive water |
| CPVC | Hot water in commercial/industrial | Handles high temps, cheaper than copper | Requires correct solvent cement; UV degrades it |
| PPR (Polypropylene) | Hot and cold water, worldwide | Very durable, smooth bore, low friction | Butt fusion joins require trained installer |
| uPVC | Cold water supply, drainage | Low cost, corrosion-resistant | Not suitable for hot water; brittle at low temps |
| Galvanised Steel | Fire protection, industrial | Strong, high pressure rated | Corrodes internally; not recommended for potable |
| Press-fit Stainless Steel | High-spec commercial, healthcare | Fast installation, hygienic | High material cost; system-specific fittings |
Common Pipe Sizing Mistakes to Avoid
Most plumbing problems on site come from a small number of recurring mistakes. Here’s what to watch for when checking a design or commissioning a new system:
Undersizing the Main Riser
Engineers sometimes correctly size all the branch pipes but undersize the main riser that feeds them. The riser carries the combined load of all floors — use the total LU for the whole building, not just one floor.
Ignoring Simultaneous Demand
Sizing every pipe for 100% simultaneous use results in massively oversized pipework and stagnation problems. Always apply the simultaneous demand factor from BS EN 806-3 or your applicable standard.
Not Accounting for Long Pipe Runs
Every metre of pipe causes friction loss. On a building with a 30-metre riser and long horizontal runs, you can easily lose 15–20 kPa before the water reaches the fixture — leaving you below minimum required pressure. Always check friction loss on the critical circuit (the longest or most loaded run).
Forgetting Fittings and Valves
Bends, tees, isolation valves, and strainers all add resistance equivalent to additional pipe length. Add 10–20% to your calculated pipe length (equivalent length method) to account for fitting losses, or calculate them individually.
Wrong Velocity for the Pipe Location
The maximum velocity rules are different for different pipe types. Suction pipes on pump systems should stay below 1.5 m/s. Discharge pipes can go to 2.5–3.0 m/s. Fixture branches should stay below 3.0 m/s to prevent noise and water hammer.
| Check our full Plumbing PPM Checklist to maintain pipe systems after installation. Plumbing Preventive Maintenance Checklist → |
Pipe Sizing Standards to Reference by Region
Different countries and regions use different standards for plumbing design. The methodology is similar, but the Loading Unit values and simultaneous demand factors may vary:
• BS EN 806 (Europe, UK, Middle East, many Commonwealth countries) — Parts 1–5 cover design, installation, operation, and maintenance of water supply systems inside buildings
• CIBSE Guide G — Public Health Engineering (UK and international) — practical design guidance aligned with BS EN 806
• ASHRAE Handbook — Plumbing Systems (North America) — uses fixture unit method with different LU values
• IPC / UPC (International Plumbing Code / Uniform Plumbing Code — North America) — widely adopted reference for pipe sizing in US projects
• NBC India (National Building Code of India 2016, Part 9) — covers plumbing design for Indian projects
• Local authority requirements — always check with the applicable local authority. In the UAE, DEWA (Dubai) and ADDC/AADC (Abu Dhabi) publish specific requirements for water supply systems
Frequently Asked Questions
What is the correct velocity for water in a plumbing pipe?
For cold water branch pipes in commercial buildings, target 1.0–2.0 m/s. Main risers can run at 1.5–3.0 m/s. Fixture connections should stay below 3.0 m/s to prevent noise and erosion. For hot water systems, keep velocity below 2.0 m/s to reduce the risk of erosion at pipe joints over time.
How do I calculate the pipe size if I don’t have the Loading Unit chart?
If you know the flow rate directly (from a fixture schedule or specification), use the formula D = √(4Q / π·V) × 1000, where Q is in m³/s and V is your target velocity in m/s. This gives you the minimum internal diameter in millimetres. Always round up to the next standard commercial pipe size.
What pipe size should I use for a single toilet?
A single WC with a cistern flush requires a 15 mm supply pipe at minimum. If you’re using a flush valve (flushometer type) rather than a cistern, the minimum is 25 mm due to the high instantaneous flow demand. Always check the fixture manufacturer’s specification for minimum supply pipe size and pressure requirements.
What is the difference between nominal pipe size and internal diameter?
Nominal pipe size (NPS or DN) is a commercial designation — not the actual measured diameter. A 50 mm DN pipe does not have exactly 50 mm internal bore. The actual internal diameter depends on the pipe material and wall thickness (schedule). Always use the actual internal bore for flow calculations, not the nominal size. Pipe manufacturer datasheets list internal bore for each pipe type and schedule.
How does pipe sizing differ for hot water versus cold water?
The calculation method is the same, but hot water systems have additional design considerations. Hot water pipes experience thermal expansion, so allowance for movement must be built in. Hot water velocity is typically kept slightly lower than cold (below 2.0 m/s) to reduce erosion risk. Heat loss from long pipe runs must also be calculated to ensure water arrives at the required temperature — this affects pipe insulation specification as much as pipe size.
Conclusion
Pipe sizing for plumbing systems isn’t as complicated as it first looks — once you understand the three-step logic of load → flow rate → diameter, the rest follows naturally. The key is to always check your work: verify the velocity, check the friction loss on the critical circuit, and confirm minimum residual pressure at the furthest fixture.
Get these right and the system will work quietly, efficiently, and without the pressure complaints that drive facilities managers and building occupants mad.
For a complete plumbing maintenance schedule to use after installation, see our Plumbing PPM Checklist → . You can also use the PPM Planner tool → to build a full annual maintenance schedule for your building’s plumbing system.