Irrigation Schedule Calculator

All four multiply together rather than one dominating the others -- Plant Type ranges 0.5x to 1.5x (a 3-to-1 spread), Climate Zone ranges 0.7x to 1.3x, Soil Type ranges 0.8x to 1.3x, and Sun Exposure ranges 0.8x to 1.2x. Because their spreads are comparably wide, getting any one of them badly wrong shifts Gallons per Week by a meaningful amount, so it's worth setting all four as accurately as you can rather than treating one as more important.

Soil Type does two things: it adjusts the weekly water-depth multiplier (sandy soil needs more total water because it drains fast) AND it sets Watering Sessions per Week directly (sandy gets 4 sessions, clay gets 2), since faster-draining soil needs more frequent, lighter watering to avoid runoff and to keep pace with drainage.

Rain and irrigation are the same inches of water to a plant, so whatever fell this week is subtracted from the weekly water demand and only the shortfall -- Supplemental Water Needed -- has to be irrigated. Every gallon, minute, and cost figure follows from that shortfall. The subtraction is floored at zero: a week wetter than the plants needed doesn't bank credit toward next week, and Rainfall Credit likewise caps at the demand rather than counting surplus that ran off or drained away. Rain changes how MUCH you water, not how often: Watering Sessions per Week stays set by Soil Type, because that reflects how fast the soil drains, so you get the same number of shorter sessions. The one exception is a week where rain met the entire demand, which leaves nothing to schedule and reports 0 sessions -- skip watering and check again next week.

Garden Area is a direct multiplier on Gallons per Week and everything derived from it -- gallons per session and the resulting minutes per session for each watering method -- but it does not change Watering Sessions per Week, which is set entirely by Soil Type.

Rainfall is now accounted for -- enter it in Rainfall This Week and it is subtracted from the weekly demand. What the calculator still can't see is your garden's actual soil infiltration rate, how much of a heavy rain soaked in rather than running off, mulch coverage, or microclimate variation within the bed -- all of which change real watering needs. Treat the output as a starting schedule to refine by checking soil moisture directly, not a precise site-specific irrigation prescription. Est. Monthly Water Cost also assumes a flat rate of $5 per CCF (748 gallons, a common water-utility billing unit) applied to Monthly Water Usage -- real water rates vary widely by utility and often use tiered pricing that charges more per gallon as usage rises, so check your own water bill's actual rate for a precise figure rather than relying on this flat-rate estimate.