If you find this program useful, helpful, cost effective, or beneficial, please feel free to buy me a few beers

Note: This is a very simplified method of calculating a pipe size, and takes no account of losses such as entry and exit losses, and losses due to bends and fittings. These losses are assumed to be compensated for, by allowing water to rise up a bit into the down pipe or catch pit.
The proper method of designing storm water pipe systems is based on the Hydraulic Grade line.


Water always flows down hill. That is, from areas of high water level to areas of low water level.
It cannot go the otherway. Also if there is a difference in water levels, water must flow.

We talk in terms of energy, and for the puposes of calculation, energy is converted to a height of water, called "head" of water. So, higher water level = higher energy.

As water flows along, it loses energy, known as a head loss. Head is lost at entry and exit, and at bends and junctions, and by friction with the pipe walls etc. Some of the initial evergy (head) is converted to the velocity of the flow. This is called velocity head and = V^2/2*g
A line connecting the water level (head) at each point in the system is called the hydraulic grade line.

Head loss, divided by length of pipe between upstream and downstream water levels, is called the hydraulic grade.
Dividing the length by the head loss, will give the hydraulic grade in the format of 1:?.
Now imagine two buckets connected by a flexible hose, the higher bucket will always drain to the lower bucket.
If the water levels are equal, no water will flow.
And get this, the higher one bucket is above the other, the greater the flow. And to add insult it injury, it makes no difference what the connecting pipe does. It can go up, or down, or around in circles, and as long as the length and diameter remain the same, the interconnecting pipe grade has no effect on the flow.
Assuming the pipe diameter remains the same, the only thing that will effect the flow, is the difference in the water level between the entry and exit, and the total length of pipe in between. That is, the hydraulic grade.
Hydraulic Grade = HeadLoss / length. (or to get it in the format 1 in ? HG = L/HL)
The hydraulic grade line is also the level to which water will rise in any connection to the pipe at that location.
So if you have a situation where water drains into a tank, or river, or lake, below the water level, or you want to take advantage of the height of the Down pipe, it is the hydraulic grade, not the pipe grade, that you should enter into the above calculator.

LOSSES The amount of head loss (HL) is dependant on the velocity of flow, and is of the form, HL = k*V^2/(2*g)
Where k is a coefficient depending on the type of bend, fitting, entry etc.
V = Velocity in m/s, and g = acceleration due to gravity = 9.81 m/s
By the way, if you are into serious pipe design you should get hold of Some manufacturers design guidelines
eg Vinedex design guidelines PE Pipes
Vinidex PVC pipes design guidlines and everything else.
This tells you all you need to know, (and a lot of things you don't need to know,) and were afraid to ask.

Anyway, for head loss at entry, k is about 0.5, and at exit, k is about 1, bends about 0.2 to 0.5. So lets see the effect of this, if velocity is 1.5m/s (which is about average)
For a pipe section with 2 bends, k=0.5+2*0.5+1= 2.5, therefore HL=2.5*1.5^2/(2*9.81) = 0.287m = 287mm

If you used the 'actual' pipe grade in the calculator above, this would be the height to which the water would rise above the pipe top, at the upstream end.
However, if you join many sections of pipe together, these loses would add up, until eventually water will come out somewhere.

Large systems are designed by starting at the outlet and working back, calculating the water level at all points, and adjusting pipe sizes to ensure flooding does not occur.
In other words, the height (RL) of the hydraulic grade line, at any opening to the pipeline, should be below the ground surface level at that point, to ensure that flooding does not occur.

For our purposes, for small building sites, we can make life easier if we add up all the minior losses first, (by assuming a velocity say 1m/ses). So from above, lets say we have 300mm of losses due to entrys, exits, bends and fittings, And lets say we have 1m of water level difference between the entry WL and the exit WL. So the available head we have to play with is 1m-0.3m = 0.7m. Divide this by the lenght of pipe, and we have what we call the available hydraulic grade.
This is what we use in our pipe design formulas. Now once we choose a pipe, we must check the velocity to see if our original velocity assumption was near enough. It can be a trial and error process for a complicated job where there is not much head to play with.
What if we use this method and find that we get a smaller DP size than the DP calculator?
A DP size is more dependant on the perimeter of the pipe (circular weir) at the entrance, than the cross sectional area. (dia) So all we need to do, is enlarge our pipe at the entrance, to the diameter required by the downpipe calculator.
Note: What we have done is designed something like a syphonic system, as the grade of the connecting pipe doesn't matter. (Don't use this to design syphonic systems though, as a syphonic system needs special fittings to get it started, and also to remove entraped air.)