Storage is sized by the shortfall, not by the day
Most tank sizing starts with daily demand and days of reserve, which is the right question when the source is a delivery or a slow well you can run for hours — that is what the cistern sizing calculator does. A spring poses a different problem, because it never stops and it is never fast. The daily total is usually fine. The instantaneous rate never is.
So the arithmetic here is a deficit: (peak draw − inflow) × minutes × events. A 6 GPM shower against a 0.75 GPM spring runs a shortfall of 5.25 GPM, so twenty minutes costs 105 gallons out of storage. Three of those in a day is 315 gallons of usable volume, and at 90 per cent usable that is a 350-gallon tank — on a source delivering 1,080 gallons a day.
Increase the tank and nothing about the spring changes; you have simply bought more simultaneous draw. Increase the draw and the tank grows linearly with it. That relationship is the one worth understanding before buying anything.
Head, in the only unit that matters
A foot of water exerts 0.433 psi at its base, and this is not a rule of thumb — it falls straight out of the density of water. Water weighs about 62.4 pounds per cubic foot, a cubic foot of base is 144 square inches, and 62.4 ÷ 144 = 0.4335 pounds per square inch per foot of depth. Sixty feet of fall is 26 psi, and that is what you have before the pipe takes its cut.
It is also why gravity systems are sited by walking the hill with a level rather than by looking at a map. The length of the pipe does not make pressure; only the vertical drop does. A 900-foot run down a gentle slope with 20 feet of fall gives 8.7 psi at the bottom, and no amount of pipe changes that number.
The pipe question is inverted, and that matters
Household plumbing asks: given this flow, what pressure is lost? That is the water flow pressure drop calculator, and it is the right question when a pump sets the flow.
A gravity line does not work that way. Nobody sets the flow — you open the valve and the line finds the flow at which friction exactly consumes the available head. So the useful question is the other one: given this fall and this pipe, what flow comes out? That is the same Hazen-Williams relation solved for Q instead of for head loss, and it is what the flow rows on this page do.
The consequence is the size table, and its shape is worth stating precisely because it is not what people expect. Friction falls roughly as the fifth power of diameter; solve the same relation for flow and deliverable flow rises as diameter to the power 2.63. That exponent is fixed, so each step up the nominal sizes multiplies the flow by the same factor whatever your fall and length are — between about 1.5 and 2.8 times per step across the sizes in the table, set purely by the ratio of the bores. There is no size at which going up stops helping.
What changes is whether the extra flow is worth buying. Once the line already passes more than the peak draw you asked for, further diameter buys nothing you can use, and the money goes further at the intake or in the tank. That is the real stopping rule — the demand running out, not the physics flattening. Pipe is bought once; head is lost on every draw for as long as the system exists.
What this page refuses to say
Anything about whether the water is safe. A spring is surface-influenced by default — what falls on the ground above it arrives in it — and no arithmetic on this page has any bearing on that. Testing is a laboratory matter and the standards are set locally: your health department will say what applies where you are, and a state-certified laboratory does the sample. Spring box construction, the sanitary seal, the screened overflow and diverting surface water above the source all sit with that same department.
If the source is a stream rather than a spring, measuring it properly is a separate exercise covered by the stream flow measurement calculator, and if the fall is large enough to be worth generating from, the micro hydro power calculator takes the same head and flow figures. For a drilled well instead, the equivalent measurement is a drawdown test on the well yield and drawdown calculator.
Questions people ask
How do I measure a spring properly?
Catch all of it and time it. That normally means a pipe or a short chute at the outlet and a container of known volume, filled repeatedly and timed with a watch. The failure mode is silent: water that runs past the container, or that keeps flowing while you swap containers, never appears in the answer and you get a confident number that is too low. Measure at least three times, and measure in more than one season, because a spring in May and the same spring in September are frequently different by a factor of three or more. Design on the September figure.
Does a bigger tank mean I can take more water from the spring?
No. The tank changes when you can use water, not how much arrives. Total supply over any long period is the spring yield and nothing else. What storage buys is the ability to draw far faster than the source for a short time, and then to sit quiet while it refills. If the daily balance line on this page is negative, more storage only lengthens the time before the problem shows up, which is worth knowing but is not a solution.
How high does the tank need to be for decent pressure?
Work backwards from the pressure you want, at 0.433 psi per foot: 20 psi needs 46 feet of fall, 40 psi needs 92 feet, and those are static figures before friction takes anything. A hillside site gets this free; a flat site does not get it at all from a stand, since a practical tank stand gives a few feet and a few psi. That is why flat-site gravity systems generally end up with a pressure pump, and hillside systems do not need one.
Why is my pressure fine until someone runs a tap?
Because static pressure and flowing pressure are different numbers, and only one of them is set by the hill. With nothing moving, friction is zero and you see the full head. Open a tap and the pipe starts taking its share, which rises steeply with flow — roughly as flow to the power 1.85. On an undersized run the drop between no flow and full flow is dramatic, which is exactly the symptom people report. The size table on this page shows what a larger bore does to that gap.
Is spring water safe to drink?
This page will not answer that and no calculator can. Springs are commonly surface-influenced, meaning what lands on the ground uphill can reach them, and appearance and taste tell you nothing at all about it. The route is your local health department for what testing applies where you live, and a state-certified laboratory for the sample itself. They will also have a position on how a spring box should be built, sealed, screened and protected from surface runoff, which matters more than anything on this page.