Rainwater Harvesting Calculator & Water Tank Size

The roof-volume relation follows CGWB's Manual on Artificial Recharge of Ground Water (2007), printed pp.118–119 (attached pp.130–131). The Texas manual (2005), printed pp.8 and 29–39 (attached pp.13 and 34–44), explains first flush and supply–demand storage balance. This tool derives a daily capped balance from conservation of water; it does not reproduce the Texas monthly algorithm.

For each one-day interval, roof runoff is $Q_r=A C P$. Consistent length and volume conversions are explicit. First flush is a runoff-equivalent depth: $F=A f$. Deduct $\min(Q_r,F)$ on every wet day to obtain tank inflow $Q$. The diverter volume is independent of runoff efficiency; a small event can be diverted completely.

With usable capacity $K_u=uK$ and preceding storage $S$, overflow is $O=\max(0,S+Q-K_u)$. Water held is $H=\min(K_u,S+Q)$, delivered demand is $Y=\min(D,H)$, and closing storage is $S'=H-Y$. This is an inflow, spill, then demand convention. Full-demand reliability counts days with unmet demand below $10^{-6}$ L; volume coverage is total delivered divided by total demanded. The capacity curve repeats this balance for 13 nominal sizes from zero to twice the selected tank.

For a no-supply, no-rain outage starting full, $V_o=Nq_p T_o$ and $K_{required}=V_o/u$. Full-tank autonomy is $K_u/(Nq_p)$. Empty-to-full refill time while use continues is $K_u/(q_{refill}-Nq_p)$; refill inflow must exceed demand. When it does not, refill time is undefined and the result reads “Cannot refill while demand continues”; this is a nonblocking warning. This independent scenario is not a protected reserve or a mains top-up rule in the annual model.

• Constant daily demand, covered watertight storage, no tank evaporation or leakage, no pump or filter throughput limit.
• First flush resets on each wet day, not each separately resolved storm. Consecutive wet days each lose the selected diversion; several storms in a day share one diversion. Select this assumption deliberately.
• The first-flush default of 0.5 mm and runoff efficiency of 0.85 are editable preliminary assumptions. Texas gives variable first-flush guidance, not a universal hygiene prescription. Avoid counting first flush again in the runoff efficiency.
• NASA POWER MERRA2 PRECTOTCORR values are daily mm/day estimates integrated over one day, stored as mm. The unmodified 2025 totals are Bengaluru 824.70 mm, Johannesburg 716.31 mm and Nairobi 558.15 mm. All 365 days are present for every city. Rainfall is local solar time.
• Constructed storms divide the entered annual depth equally among wet days from the first day through the last day at the chosen interval. They do not wrap across New Year and have no claim to measured or forecast rainfall.
• January 1 starts at the selected initial fraction; no warm-up or automatic annual repetition. Initial water is included in service, but excluded from harvested volume. No rainfall is credited in the outage or refill calculations.
• Dead storage and freeboard are represented together by the usable fraction. The animated level represents usable volume fraction, not physical water depth or tank geometry.

Preliminary volumetric sizing of one roof-fed storage tank for a 365-day scenario. Daily aggregation cannot resolve storm intensity, within-day use, gutter losses or multiple first-flush resets; the spill-before-demand convention is conservative when a full tank is drawn during a storm. A 2025 scenario percentage is not a long-term reliability probability. Use local multi-year data and drought sequences for final supply design.

No current jurisdiction's regulatory sizing rule is implemented. Historical manuals are methodological references, not evidence of present legal compliance. Check the local rules and the standard in force at the project location. Groundwater recharge needs separate hydrogeological and contamination assessment. Potable use needs appropriate treatment, testing and approval. Installation, backflow protection, overflow drainage and structural work belong to a qualified person. The schematic is a process illustration, not a construction or gravity-flow layout.