Estimate Sprinkler Count Above Coverage Area?

8/19/2026

For those of you who have designed sprinkler systems in old or existing buildings, is there any common rule of thumb for how much the sprinkler count tends to increase because of irregular geometry, existing walls, small rooms, etc.?

In my case, I’m ending up with roughly 20–30% more sprinklers than what I would get from simply dividing the total protected area by the maximum allowable coverage per sprinkler.

Does that sound normal for an existing building, or would you consider that excessive?

Thanks in advance!

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6 Comments

Pete H

8/19/2026 06:01:25 am

I have no set rule of thumb for this (as half the time with irregular shaped rooms they'll be small enough you can just spot the heads), but if I was rushing I might just imagine the space at the fullest size rectangle possible and use the additional heads that'd create as my "cushion".

Dan Wilder

8/19/2026 08:13:26 am

For 225 spacing, 180-158 sq ft seems conservative on the low side but that isn't out of the ordinary with cut up floor plans. For 130, being in the 104-91 sq ft realm is in line with what I would expect. For the 100 sq ft,, 70-80 sg ft seems conservative but again, a couple beams can put that number into alignment really quickly. If you're winning the bids and making money...sounds like things are good!

So much more info needed....and we never use a max sq ft / building area (225 for LH, 130 for OH, 100 for EH & Storage)

Combustible or Noncombustible
Structure type (trusses/girders, trusses/beams, all beams, concrete tee's, TJI's)
Occupancy
Ceilings
Full height walls

Sometimes the structure automatically doubles your sprinkler count, sometimes is the occupancy. Drop ceilings with offices will nearly double things in many cases.

In a pinch we will square foot a project when providing ROM's but taking that number to a contract is rare. When in doubt, we sell the design first with the install number closely after.

Jack G

8/19/2026 08:59:17 am

For old light hazard buildings in Phillie that have window offices and are cut up my rule of thumb is to divide the building square footage by 100 and it seems to work pretty well.
If there are large open areas per floor I divide by 120 ft2.,

Jesse

8/19/2026 09:11:43 am

I've said in this forum before, and on the university side, that I generally avoid trying to max-out AS coverage as it leaves little margin for field conditions.

I don't do a lot of estimating but Dan's ideas seem workable.

Glenn Berger

8/19/2026 03:47:19 pm

Without better information - I just assume 100 sq feet will be a comfortable factor without visiting the site.

andrew j koutsoftas

8/28/2026 10:04:59 am

for the roughest of estimates:
light hazard could be figured for 200sqft and ordinary hazard at 100sqft per sprinkler. This is best for a ROM.
Then factor in as much of the building/project specifics as possible to get to a contract proposal.

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Estimate Sprinkler Count Above Coverage Area?

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