Material and nominal size go in; the true bore comes out as gallons held, water weight, feet per gallon — and the two numbers a pipe earns its keep by: how long hot water takes to arrive, and how fast the water moves.
What Your Results Mean
Two panels run side by side because a pipe answers two different questions. The volume panel is geometry: the bore's circular area times the run's length, with the one honest trap — nominal size is a name, so the calculator fills the bore field from the standard tables (ASTM B88 copper, F876 PEX, D2846 CPVC, D1785 Schedule 40) and lets you override it with a caliper reading. The delivery panel is behavior: that volume divided by your fixture's flow gives the wait for hot water, and the velocity row tells you whether the same water is moving politely or arriving at a gallop.
The bore row deserves the first glance, always. “3/4-inch” copper Type L is 0.785 inches inside; the same nominal PEX is 0.681; Schedule 40 is 0.824. Squared, those differences become capacity: the chart under the calculator shows all five materials at your selected size, and the spread is the quiet fact behind every copper-versus-PEX argument. If your pipe is old, imported or reamed-out galvanized, trust the caliper over any table — that's why the field is editable.
Wait time answers the dead-leg question: the run between heater and tap is full of cooled water, and the fixture has to push it out before anything warm arrives. Volume divided by flow, with the flow set by the fixture — an aerated faucet at 1.2–1.5 GPM, a modern shower at 2–2.5, a tub at 4–5. The annual row multiplies the purge by 365, the once-a-day habit; your house's real number scales with your household's patience.
Reading the Lines
- Bore used: the inside diameter doing the work. From the tables for standard material, or your measurement — nominal size never enters the math.
- Volume per 100 ft: the trade comparison number. One hundred feet of 1-inch copper holds what 171 feet of 3/4 does.
- Water weight: 8.34 pounds per gallon. A 100-ft run of 2-inch Schedule 40 carries 145 lb of water — hangers, not hope, hold that up.
- Feet per gallon: the inverse trade constant — 39.8 feet of 3/4 copper per gallon, useful when you're counting jugs for winterizing.
- Wait to clear: the dead-leg clock. Shorten it by shrinking volume (smaller bore, home-run layout), shrinking distance, or recirculating.
- Velocity: the verdict on speed — quiet under 5 ft/s, brisk to 8 (fine for cold, hold hot lines nearer 5), too fast above 8 where noise, hammer and copper erosion live.
What the Calculator Does Not Do
It sizes straight pipe, not systems. Fittings nudge the total honestly by less than a cubic inch each; friction loss along a long run, pump curves, pressure-tank drawdown and expansion-tank sizing are their own engineering, and drain/waste lines run partially full under a different discipline entirely. Old galvanized deserves a special warning: the table bore is for new pipe, and fifty years of scale can halve an old galvanized line — measure it or assume the worst. And winterizing antifreeze totals are for the supply side only; traps, the toilet bowl and appliance bottoms take their own pours. For the method and the tables behind every number, the pipe volume guide walks it all by hand.
Pro Tip
Time your own dead leg once with a stopwatch, then work backward: wait × flow ≈ the branch's real volume. If the arithmetic says 45 seconds and the tap takes two minutes, the pipe isn't the problem — the water heater is, and the water heater sizing calculator is the next stop.
The Math Behind the Calculator
The volume side is one formula; the delivery side is a division and a one-liner:
area = π × ID² ÷ 4 (in²)
volume = area × L × 12 (in³, L in feet)
gallons = volume ÷ 231 (231 in³ = 1 US gal, exact)
weight = gallons × 8.34 (lb)
wait = gallons ÷ GPM × 60 (seconds)
velocity = 0.4085 × GPM ÷ ID² (ft/s)
The bore comes from the dimensional standards — the same table the material selector draws from:
In gallons per 100 feet — the number the panel's second row shows — those bores become:
The velocity verdict follows field practice: comfortable below 5 ft/s in any service, brisk from 5 to 8 where hot lines want to stay (copper's erosion-corrosion risk climbs with speed and temperature), and too fast above 8 — where noise at elbows, hammer on closing valves, and wall-thinning at the outsides of hot copper bends begin. Metric mode converts lengths, volumes, weights, flows and speeds; nominal pipe sizes stay in inches in both systems, because that is how pipe is named and bought.
Assumptions & Limitations
Every number above rests on these constants. If your pipe or product differs, change the bore field — or read the linked guide for the full reasoning.
- Constants: 231 cubic inches per US gallon (exact); water at 8.34 lb/gal; 1 gallon = 3.785 L; 1 lb = 0.454 kg
- Bore tables: copper L/M per ASTM B88; PEX per F876 SDR-9 (OD less twice minimum wall — Uponor PEX-a runs ~0.01″ smaller); CPVC CTS at the mid-range of D2846 walls; PVC/CPVC Sch 40 and galvanized steel share IPS geometry per D1785
- Straight-pipe volume ignores fittings (well under a cubic inch each) and partial blockage — old galvanized can be half its table bore, so measure it
- Wait time assumes the fixture runs at the entered flow and the branch starts full of cooled water at heater-setpoint-adjacent temperature loss; recirculating systems change the problem
- Annual purge assumes one full clear per day, 365 days — scale it to your household's habits
- Velocity thresholds (5 / 8 ft/s) are field sizing practice, not a code citation; local code and the pipe manufacturer's literature own the final word
- Drain, waste and vent piping runs partially full and is sized by fixture units and slope — nothing here applies to it
- Metric mode converts m, mm, L, L/min, kg and m/s; nominal size names stay in inches, the trade standard everywhere pipe is sold
Worked Example: The 60-Foot Dead Leg
The same branch walked through in the pipe volume guide, so every number below is checkable against it. Leave the defaults as they load: 3/4-inch copper Type L, 60 feet, a 2 GPM shower asking for hot water:
- Bore: 3/4″ copper L is 0.785 in inside — 0.484 in² of area, 5.81 in³ per foot.
- Volume: 348 in³ ÷ 231 = 1.51 gallons (5.7 L), weighing 12.6 lb.
- Trade numbers: 2.51 gal per 100 ft; 39.8 ft of this pipe per gallon.
- Delivery: 1.51 ÷ 2.0 = 45 seconds to first hot water; velocity 1.33 ft/s — quiet.
Now stress it. Switch the material to PEX at the same 3/4″ nominal: the bore drops to 0.681, the run holds 1.14 gallons, and the wait drops to 34 seconds — but swap the size to 1/2″ PEX (the home-run manifold favorite) and 60 feet holds just 0.58 gal, clearing in 17 seconds, at a brisker 3.47 ft/s for the same 2 GPM. That trade — smaller pipe, faster clear, louder water — is exactly what the velocity verdict is for.
Push the flow to 5 GPM (a tub filling) in 1/2″ PEX: 8.68 ft/s, too fast. Keep the 5 GPM and switch to 3/4″ copper: 3.31 ft/s, quiet again — the same flow behaving completely differently in two “half/three-quarter” pipes. And for the winterizers: change the material to copper, set 100 feet of 1/2″ plus count your 3/4″ mains separately — the guide's example house (80 ft of 1/2″ + 40 ft of 3/4″) sums to 1.98 gallons, which is why two jugs of RV antifreeze fill the supply side of a small house.
Run your own branch before believing any of it — a stopwatch at the tap is the cheapest audit in plumbing. If the measured wait and this arithmetic disagree badly, the pipe is not what you think it is, or the water heater has an opinion; measure the bore, then size the heater.