How To Calculate The Size Of A Gutter

How To Calculate The Size Of A Gutter

gutter size calculation

Size Does Matter


With gutter replacement size really does matter! Get it wrong and there will be consequences. You could simply measure the old guttering and replace with the same. However it’s worth measuring and doing the calculations to make sure. If it’s a fairly old house the gutters may have been replaced by previous owners. Can you be sure they got it right? Is it a newer property with its original guttering?  Remember, some builders have been known to skimp on such items.


Thing is, if the size is too small the gutters will clog up more quickly so the need for gutter cleaning becomes much more frequent. Water spilling over the edge of the gutting due to clogging is a nuisance and can lead to structural problems. Water running off the roof may even miss the gutters altogether if they’re too narrow so you’ll have wasted your time and money!


Oversize guttering is no good either. It’s unsightly and you’ll also need new, larger down pipes. It all costs more than necessary and guttering replacement is expensive enough! You can’t afford to skimp but why spend more than necessary?


With plastic guttering and a simple roof design, the chances are that it will be five or six inches across and of a pattern called half round or it may be a more fancy design called “K Style”.


If your area frequently experiences very heavy rain, your house is rather larger than average or the roof is very steeply pitched; the guttering may need to be more substantial in both size and construction.


In most cases the size of guttering required is determined by the surface area of the roof. To measure this accurately, get up there with a tape measure to check the length and breadth of each slope of the roof. With a simple roof, with just two sloping sides and gable ends, the method is straightforward. For a more elaborate roof shape you measure each section and add the areas together. How hard can it be?  However, getting up there is a perilous undertaking for an amateur involving ladders or scaffolding towers and the ground below is very hard!


It’s less accurate but much safer to measure the length of the walls on the ground, not forgetting to add in the overhang of the eaves. When it comes to actually carrying out the gutter replacement, there is no alternative to climbing up to the roofline.   


At this point most of us will probably decide to call in a roofline specialist but for the dedicated DIY enthusiast, there is still the pitch of the roof to be measured and factored in to the roof area calculation because steeper roofs catch more rainwater.

How To Do It


The measurement for this can be taken inside the roof space where the only real risk is that of putting your foot through the ceiling!  Being careful to avoid doing that, take a two foot spirit level and holding it level with one end against the sloping roof measure from the one foot point Upwards to the inside of the roof. That measurement gives you the pitch of the roof i.e. 9 inches at the 12 inch mark on the level, your pitch is 9 in 12; 6 inches would be 6 in 12 and so on. Then this table will give the pitch factor for your roof; the figure at the end of each line from 1 to 1.3:


   12-in-12 or higher — 1.3

   9-in-12 to 11-in-12 — 1.2

   6-in-12 to 8-in-12 — 1.1

   4-in-12 to 5-in-12 — 1.05

   Flat to 3-in-12 — 1


Multiply the surface area by the pitch factor to find the rain collection area for your roof and you can buy the right size guttering. In plastic K Style, 5 inch guttering will drain up to 5520 ft², 6 inch up to 7960 ft². In Half Round the capacities are 2500ft² for 5 inch and for 6 inch, 3840ft².  


Plastic gutter repair usually means renewing perished seals or replacing a damaged section. Cast iron gutters can often be patched using sealants so gutter repair may be worth considering.




Whether gutter replacement is called for or gutter repair, to do either properly is not a simple task. That’s what professional gutter repair specialists are there for. All you have to do is choose the right one.    



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