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Use our Rainwater Drainage Calculator to quickly find runoff flow. Simple tool to calculate rainwater drainage, runoff, and storm drain size.
Rain can feel refreshing, but when it pours, water needs a way out. Too much water on roofs, yards, or streets can cause big trouble. That’s where a rainwater drainage calculator comes in handy. We’ve built this tool so you can find runoff flow in seconds.
It’s simple. You just enter the drainage area, rainfall rate, and surface type. The calculator shows you the water flow right away.
The formula for rainwater runoff is easy to follow. Engineers use the Rational Method. The formula is:
Q = C × i × A
Think of it like this: the harder it rains, the bigger the area, and the smoother the surface, the more water you get.
Let’s look at a quick example.
Say you have a 200 m² roof. The rainfall intensity is 100 mm/hr, and the coefficient is 0.9 (typical for smooth roofs).
Plug it in:
Q = 0.9 × 100 × 200 × 2.77778×10⁻⁷
Q = 0.005 m³/s
That equals 5.0 L/s or 0.176 cfs.
So, your roof sends about five liters of water per second during that storm.
Once you know the flow, you can size the drain. Compare your Q value with drain capacity charts. For example, one downspout can handle a certain flow, and gutters have their own limits.
If your Q is bigger than what one drain can handle, add another drain or use a larger one. It’s like planning how many doors you need for people to exit a hall.
Depth depends on soil, slope, and frost line. In most cases, drains sit below ground level, with enough slope so water flows by gravity. Always check local building codes. A shallow drain might clog or freeze, while a deep one works better but costs more.
Using our tool is as easy as pie:
The result also shows alternate units, so you don’t need to convert anything.
Rainwater runoff may sound tricky, but with the right tool, it’s simple. Our rainwater drainage calculator saves you time and helps you make smart choices. Whether you’re fixing a roof, planning a garden, or designing a storm system, this tool gives you clear answers fast.
It’s an online tool that calculates how much rainwater flows off a surface.
Runoff helps you size gutters, pipes, and drains to avoid flooding.
Yes, you can use it for roofs, yards, and small plots.
It’s a number between 0 and 1 that shows how much water runs off. A smooth roof is high (about 0.9). A grassy yard is low (about 0.3).
Yes, every city has its own rainfall design values. Check local data or use common values.