Watering the Garden: Supply, Storage and Demand

Garden water has three separate numbers, and they rarely match: how much arrives, how much you can hold, and how much the garden wants. These tools measure each, and the interesting part is always the mismatch.

What actually comes off a roof

The Rain Barrel Capacity Calculator produces the number that surprises everyone.

A modest 40 × 30 ft roof, one inch of rain, at 85% collection efficiency:

Water off the roof	636 gallons
What one 55-gallon barrel holds	55
Overflow	581 gallons
Barrels needed to catch it all	12

One inch of rain. One roof. Twelve barrels to catch a single ordinary rainfall.

That reframes what a rain barrel is for. It is not a way to capture your rainfall — it is a way to capture a useful sample of it. The 636-gallon figure is also worth knowing for the opposite reason: that is what is currently arriving at your foundations every time it rains an inch.

And the payoff: those 55 gallons cover 2.8 weeks of a 20-gallon-a-week habit. One barrel is small against the rain and large against the watering can.

What the garden actually wants

The Lawn & Garden Watering Calculator works the demand side, and its key move is subtracting the rain you already got.

A 1,000 sq ft area, aiming for 1 inch a week, with 0.25 inches of rain already fallen:

Still needed this week	0.75 in
Gallons that is	468
Minutes per session (2 sessions)	45
Total for the week	90 min

Every inch over 1,000 sq ft is 623 gallons. That is the conversion worth remembering, because "an inch a week" sounds like nothing and is several hundred gallons.

⚠️ Measure your sprinkler rather than trusting its rating. Put a few straight-sided tins around the zone, run it for 15 minutes, measure the depth and multiply by four. Sprinkler output varies with pressure, and pressure varies with what else is running — the rated figure is a starting guess, not your garden's number.

What your tap can actually supply

The Drip Irrigation Zone Calculator covers the constraint people meet last and should meet first: the supply.

Measure by timing a bucket — 5 gallons in 45 seconds:

Your measured supply	6.67 gpm (400 gph)
What 200 emitters would demand	400 gph
Emitters one zone can run	160
Zones needed	2

The demand and the supply are identical at 400 gph — and that is exactly why it doesn't work. With a 20% safety margin the honest figure is 160 emitters a zone, so the system splits into two zones running in sequence: 120 minutes for a full cycle instead of 60.

⚠️ A drip system that exceeds its supply does not fail loudly. Pressure drops, and the emitters furthest from the tap quietly deliver less than the ones nearest it. Everything looks like it is running. The plants at the end of the line are the ones that tell you, weeks later.

Time the bucket. It takes a minute and it is the only figure in the whole system that is genuinely yours.

The three numbers together

Arrives	636 gal per inch, off a small roof
Stored	55 gal per barrel — 2.8 weeks of hand watering
Wanted	623 gal per inch, per 1,000 sq ft

Notice that the roof and the garden are nearly the same size of number. An inch of rain on the roof is roughly an inch of watering for a similar area — which is a useful sanity check, and a reminder that storage, not supply, is the binding constraint.

The order to work in

1. Measure the supply first — time the bucket. Everything downstream is capped by it.
2. Work out the demand — area × depth, minus rain already fallen.
3. Then size storage, knowing it will only ever catch a fraction.

Doing it the other way round — buying barrels, then a drip kit, then discovering the tap — is how people end up with a system that runs at half pressure and a barrel that overflows in every storm.

⚠️ Deep and infrequent beats little and often. A shallow daily sprinkle keeps roots near the surface where they dry out fastest. The tool defaults to two sessions a week for that reason: it is aiming to wet the soil profile, not the surface.