Your cistern’s size should match the water you use and the reserve you need between supplies. More capacity can give you additional time if a delivery is delayed, but it also leaves more water in storage when use is low.
Calculate the usable capacity you need
Start with measured daily consumption and the number of days of coverage you want to plan for. Include your chosen reserve for possible delays in those days.
Required usable capacity (liters) = daily consumption (liters/day) × days of coverage
For a hypothetical property using 500 liters daily and planning for seven days, the calculation is 500 × 7 = 3,500 usable liters. These are example inputs, not recommended consumption or reserve figures for every Cabo home.
The cistern calculator uses the daily consumption and coverage days you enter. You can also compare the result with your current usable capacity, if known. It does not measure the tank’s dimensions or establish water safety.
Measure use before choosing a tank
Use meter readings or reliable records of volume and the period of use. Account for occupancy changes and, where possible, separate irrigation, pool replenishment and leaks. The consumption guide helps organize those records.
If you only have estimates, identify the assumptions and compare several scenarios. A pool does not add a fixed number of liters per day; use the property’s actual replenishment records.
Total and usable volume can differ
The tank’s advertised capacity does not necessarily describe how much water the installation can supply. Confirm the intended fill level, intake position and pump operating requirements.
When buying or building, have the installer document usable capacity and review the structure, access, connections and maintenance provisions. Consult the materials guide as well.
Choose a reserve
Review your supply history and current delivery lead times. Decide which uses you need to maintain during a delay and which you can temporarily reduce. For a vacation property, compare occupied and vacant periods.
Reserve is a planning decision. No fixed duration guarantees coverage through every interruption. If you have already included reserve in the coverage days, do not add it again to the result.
Coverage is not a water-quality test
Estimated days of coverage = current usable capacity ÷ daily consumption
This ratio estimates how long the volume would last under the assumed use, without new supply. It does not identify the age of every portion of water; filling, mixing and flow through the tank also matter.
The EPA explains that oversized storage and poor circulation can increase water age. Use the calculation to review operation, without treating seven, fourteen or twenty-one days as universal drinking-water safety limits.
Compare delivery and maintenance
Request quotations for volumes you can actually receive. Check transport charges, access requirements and whether partial deliveries are available. Do not assume every larger order will have a lower price per liter.
A larger cistern also needs space and access for service. Two tanks may allow part of the installation to be isolated, but they need suitable design and operation. Having two tanks does not automatically halve water age.
Frequently asked questions
Is a 20,000-liter cistern too large? The number alone does not answer that. Compare usable capacity with measured consumption and chosen coverage, then have the design reviewed before building or modifying it.
Can I request partial fills? Confirm availability and price with the supplier. The volume must still meet the installation’s operating requirements and your reserve needs.
Does the calculator choose my reserve? No. You enter coverage days, including your chosen reserve. The result is an arithmetic estimate to discuss with the installer and water supplier.
What changes when the home is vacant? Use may fall substantially. Plan supply and a review before occupants return; do not use peak-season consumption to represent the entire year.
Compare pipa quotations and keep your maintenance history alongside the calculations.