Pipe Flow Velocity Calculator
Sump and canister plumbing lives or dies by one number: how fast the water moves through the pipe. Enter your flow rate and pipe size to get velocity in metres and feet per second, plus a verdict against the velocity ranges sump builders use for quiet gravity drains and efficient pump returns.
Calculate pipe velocity
v = Q ÷ A. IDs are US Schedule 40; metric PVC and flexible hose differ — measure and use Custom if unsure. Always size from inner diameter, not the nominal or outer size.
The formula
Velocity is just flow divided by the pipe’s cross-sectional area — the continuity equation:
Because area scales with the square of diameter, small pipe-size steps make big velocity differences: moving from 1″ to 1½″ Schedule 40 pipe cuts velocity (and roughly quadratically, friction loss) by more than half at the same flow. That is why experienced sump builders’ advice is so often simply “go up one size”.
Why velocity targets differ for drains and returns
Gravity drains have no pump pushing them — only the height of water above the outlet. Run them fast and they flirt with a full siphon: the pipe alternately gulps air and water, producing the flushing and gurgling that plagues badly plumbed sumps, and leaving no reserve capacity if snails or biofilm narrow the bore. The widely used guideline in the reef and sump-building community is to size gravity drains for about 2 ft/s (0.6 m/s) or less, and quiet designs (Durso, Herbie, Bean Animal standpipes) assume that kind of margin.
Pump-fed returns are the opposite trade: the pump provides pressure, so higher velocity is workable, but friction loss grows with roughly the square of velocity — every extra foot per second costs pump head, watts and eventually noise. General small-plumbing practice lands returns in the 3–6 ft/s (0.9–1.8 m/s) band, with ~8 ft/s treated as the ceiling before hammer and hiss become obnoxious.
Common mistakes
- Sizing by nominal or outer diameter. A “1-inch” Sch 40 pipe has a 26.6 mm (1.05″) bore; flexible hose is usually named by ID but not always. Measure.
- Using the pump’s rated flow. Rated GPH is at zero head. After 1.5 m of vertical rise and a few elbows, real flow is commonly 50–70% of the label — our filter flow calculator covers this.
- Matching the drain to the return size. The drain should normally be larger than the return line, because it runs at a fraction of the velocity budget and must swallow the full return flow with margin.
- Forgetting elbows. Each 90° elbow adds friction equivalent to roughly a metre of straight pipe at these sizes; two 45s beat one 90.
Frequently asked questions
Why does my drain gurgle and surge?
Classic symptom of a drain running too close to full-siphon capacity: it alternates between siphoning (fast) and gulping air (slow). Fixes, in order of effort: tune or fit a proper standpipe (Durso/Herbie), throttle the return pump slightly, or upsize the drain so velocity falls under ~2 ft/s. A gate valve on a Herbie-style drain gives the finest control.
Can a pipe be too big?
Hydraulically almost never for a drain — oversize just means slower, quieter, more clog-tolerant. The real costs are money, bulkhead hole size and bulk. For pressure-side returns, extreme oversizing wastes nothing energetically but can make flow so lazy that detritus settles in horizontal runs; keeping returns near 3 ft/s avoids that.
How do I account for head loss and elbows properly?
Full treatment uses the Darcy–Weisbach equation with fitting equivalent lengths — overkill for most builds. The hobby shortcut: take vertical rise, add ~1 m of equivalent length per 90° elbow, and read real output from your pump’s head-flow curve at that total. If the answer lands between pipe sizes, choose the bigger pipe.
Does this apply to canister filter hoses too?
Yes — the same v = Q/A maths explains why a 16/22 mm hose canister moves noticeably more water than a 12/16 mm one at similar pump power, and why kinks (which slash effective ID) hurt so much. Keep hose runs short, sweeping and kink-free, and clean the bore; biofilm a millimetre thick measurably cuts flow in small hoses.