SettingsSettings for this calculationUS
The total volume of water held in the boiler/hydronic system's piping, boiler, and terminal units.
This includes the boiler itself, all distribution piping, and terminal units (radiators, coils, etc.) — often available from system design documents or estimated from pipe volume calculations.
The expected fraction of total system water volume lost per period from minor leaks, air venting, and maintenance draindowns.
A well-sealed, well-maintained closed system typically loses only a small percentage of its volume over a given period. Older or leakier systems will have a higher effective loss rate.
Estimated makeup water needed
10.6 gal
Loss rate percentage varies widely by system age, condition, and maintenance practice — an unusually high or increasing makeup water demand can indicate a leak that should be investigated rather than simply compensated for.
They open the calculator with your figures already in it
Boiler Makeup Water Calculator: 10.6 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Makeup water rate = system water volume × estimated loss rate percentage, a standard method for estimating the water treatment/makeup demand from typical system losses (minor leaks, air venting, maintenance draindowns)
Inputs used
- Total System Water Volume
- 530 gal
- Estimated Loss Rate (% of System Volume per Period)
- 2
Confidence note: Loss rate percentage varies widely by system age, condition, and maintenance practice — an unusually high or increasing makeup water demand can indicate a leak that should be investigated rather than simply compensated for.
What this calculation does not cover
- The whole calculation is one multiplication of system volume by the loss percentage, so operating pressure, fill-loop and feed water temperature, and the behaviour of the expansion vessel never enter it — a system that cycles pressure hard and one that sits stable return the same volume if you type the same two numbers.
- Loss rate is entered as a percentage "per period" but no period is ever asked for and the answer comes back as a plain volume, so the figure carries whatever window you had in mind — annual, heating-season or monthly — and a figure quoted on one window cannot be compared with a system measured on another.
- The loss-rate box accepts 0.5 to 5 and rewrites anything outside that band to the nearest bound when you leave the field, showing a notice such as "Using 5 — the highest this calculator accepts. You entered 10", so a system genuinely shedding 8 percent has to be modelled by scaling the volume figure instead, and the 0.5 floor means a tight sealed system cannot be expressed as needing nothing.
- System water volume is taken exactly as supplied and is never derived from or checked against pipe runs, boiler content or terminal-unit capacity, so a volume estimated loosely from design drawings passes into the makeup figure one-for-one — understate the system by a fifth and the water is understated by a fifth.
- The result arrives as a single bulk volume with no breakdown behind it, so the share going to air venting, to maintenance draindowns and to actual leakage stays merged in one number even though the loss-rate help text names all three as separate causes, and it will not tell you which of them to chase.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-05 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Makeup water rate = system water volume × estimated loss rate percentage, a standard method for estimating the water treatment/makeup demand from typical system losses (minor leaks, air venting, maintenance draindowns)
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