The one number nobody can give you
Every runoff calculation starts with a rainfall depth, and that depth is specific to your location and to the storm you have been asked to design for. It is not a national figure and it is not a seasonal average. It comes from rainfall frequency data for your coordinates, for a stated return period and duration, and the return period and duration are chosen by whoever is reviewing the work. Two properties in the same county can be given different figures for different reasons.
This page therefore requires the rainfall depth as an input and will not compute anything without it. That is deliberate. A tool that quietly supplies a plausible-looking rainfall figure produces a plausible-looking answer that is wrong in a way nobody notices until the water arrives.
What a runoff coefficient is doing in the arithmetic
The volume that runs off a surface during a storm is the area times the depth of rain times a coefficient, and the coefficient is the fraction of the rain that leaves as surface flow rather than soaking in, evaporating, or sitting in surface depressions. A sealed surface passes most of it on. Vegetated ground passes a small fraction. Everything else is in between, and where in between depends on soil, slope, antecedent wetness and how compacted the ground is.
Permeable paving works by moving the coefficient down. It does not move it to zero, and modelling it as zero is the single most common mistake in this calculation. A permeable system still sheds water in a storm that exceeds its infiltration rate, still sheds water once the base is full, and sheds progressively more as the surface clogs with the fines that every site produces. The coefficient you enter should be the one your local drainage manual permits you to claim for the system as built, not the best-case figure from a product sheet.
The base has to hold what the surface does not shed
If the surface only passes twenty percent of the rain on as runoff, the other eighty percent went somewhere, and in a permeable system it went into the open-graded stone underneath. That layer stores water in the voids between the aggregate, which is why it must be clean open-graded stone rather than the dense-graded base used under conventional paving. Dense-graded material is designed to have its voids filled, and a base full of fines stores almost nothing.
The depth needed follows directly: the volume to store, divided by the area, divided by the void ratio. The void ratio is a property of the specific gradation you buy and comes from the supplier. The arithmetic is easy and the trap is elsewhere, in what else drains onto the paving. A roof or a sloping driveway discharging onto a permeable patio adds its own catchment to the volume while adding no area to store it in, and the storage depth climbs quickly. That is the usual reason a system that looked fine on paper is under water an hour into a storm.
Storage depth and structural depth are different questions
The depth this page gives you is the depth needed to hold water. It is not the depth needed to carry load. A patio crossed only on foot may need less structure than storage; a driveway crossed by vehicles will usually need more structure than storage. Whichever requirement is deeper is the one you build, and the structural side depends on the soil beneath and on what will drive over it. Compaction is a complication here too, because the structural instinct is to compact hard and the storage function wants the voids intact, which is one of several reasons permeable systems are built to their own specification rather than a modified conventional one.
None of that can be judged from a page. What can be said plainly is that a base that fails under a permeable surface is a full rebuild, that the failure mode is usually water rather than load, and that the subgrade below has to accept water at a rate measured on site. A storage layer over ground that will not drain is a buried tank with paving on the lid.
The permission question, and where this fits
Whether the work needs review, what storm you must design to, what coefficients you may claim, whether the property has a limit on hard surface coverage, and whether replacing an existing surface counts as new impervious area are all local determinations. Adding a driveway or a large patio commonly does trigger one of them. The local building department or stormwater authority is the source, and the answer sometimes changes the design substantially. Ask before the design is finished rather than after.
This page works in volume: how much water, over the whole storm. If what you need instead is a peak flow rate in cubic feet per second, for sizing a swale or a pipe, that is the culvert and swale flow calculator, which uses the same coefficients and a rainfall intensity rather than a depth. For the paving above the base, see the paver patio calculator and the cut waste calculator, and for a subsurface trench that collects rather than stores, the french drain calculator. Where the water goes across the surface before it gets anywhere is the hardscape slope calculator.
Questions people ask
Where do I get the rainfall depth to enter?
From rainfall frequency data for your specific location, for the return period and storm duration your jurisdiction requires, which the local stormwater authority, road authority or a civil engineer can tell you. The two parts matter equally: a two-year one-hour storm and a hundred-year twenty-four-hour storm at the same address are very different depths, and using the wrong one produces an answer that is confidently wrong. If nobody has told you what to design for, that conversation is the first step, not the calculation.
Does permeable paving eliminate runoff?
No, and treating it as if it does is how these systems get undersized. It reduces the fraction of rainfall that leaves as surface flow, typically by a large amount in modest storms and by much less in large ones. Once the storage layer fills, or once rain falls faster than the surface can accept it, the surface behaves progressively more like a conventional one. It also degrades: fines work into the joints and the pores over time, and unless the maintenance actually happens, the coefficient drifts upward year on year.
What void ratio should I use for the stone?
Ask the supplier for the gradation you are buying, because it varies with the size and uniformity of the aggregate and with how it is placed. Clean, uniformly sized open-graded stone holds a substantial fraction of its volume as void space; anything with fines in it holds much less. The important discipline is not the exact figure but making sure the material you get is genuinely the open-graded product you priced. A load of dense-graded base delivered instead will look similar in the pile and will store almost nothing.
Can I run my downspouts onto a permeable patio?
Only if the storage was sized for it, which is what the run-on field here is for. A roof is a highly efficient collector and a modest roof area can easily deliver more water than the patio itself receives. Directing it onto the paving without accounting for it fills the base early in the storm and the surface then behaves like any other hard surface, usually while it is raining hardest. It also concentrates fines and debris off the roof at the discharge point, which is where clogging starts.
Is the storage depth the same as the base depth I should build?
Not necessarily, and you build the deeper of the two requirements. Storage depth comes from the water volume, which this page gives you. Structural depth comes from the soil and the load crossing the surface, which it does not, because that depends on things only an on-site assessment can establish. For a foot-traffic patio the storage requirement often governs; for anything carrying vehicles the structural requirement usually does, and by a wide margin.