Watermaker / Desalination Sizing Calculator
Calculate the ideal watermaker capacity for your yacht or boat based on the number of people on board, daily water consumption, passage length, tank size, and a safety reserve margin.
Watermaker Sizing Results
How do I size a watermaker for my yacht?
Watermaker sizing starts with your total daily fresh water demand. Multiply the number of people on board by the expected water consumption per person per day, then add any non-personal demand such as deck washing or hot tub refills.
$$Daily\;Demand = (People \times Liters\;Per\;Person) + Extra\;Daily\;Demand$$
How is watermaker capacity calculated?
The watermaker must produce enough fresh water per day to meet total demand, plus a reserve margin for peak usage or membrane degradation. Dividing the required daily output by the number of hours the unit will run gives the minimum hourly production rate.
$$Hourly\;Rate = \frac{Daily\;Demand \times (1 + Reserve\;\% / 100)} {Run\;Hours\;Per\;Day}$$
Why does sea water temperature matter?
Reverse osmosis membranes are rated at a standard temperature, typically 25 °C (77 °F). In colder water, the membrane's permeability decreases and actual output drops. As a general rule, output falls approximately 2–3% for every 1 °C below 25 °C. In cold North Atlantic or Scandinavian waters at 8–12 °C, a watermaker may only produce 60–70% of its rated capacity.
This calculator applies an approximate correction factor: 1.0× for warm tropical waters, 1.2× for temperate waters, and 1.4× for cold waters, so you select a unit rated high enough to compensate.
How much water does a yacht use per day?
Daily water consumption varies enormously depending on the yacht type and lifestyle. Economy cruisers may use 60–100 liters per person per day, while luxury superyachts with full laundry, jacuzzis, and multiple showers can consume 250–400 liters per person per day or more.
Non-personal uses such as deck washing, tender cleaning, ice-makers, and hot tub refills can add 50–200+ liters per day to the total.
How many hours should a watermaker run per day?
Most marine watermakers are designed for intermittent use rather than continuous 24-hour operation. Cruising yachts typically run their watermaker 4–8 hours per day, while larger superyachts may run theirs 10–16 hours. Running fewer hours requires a higher-capacity unit, while longer run times allow a smaller unit to meet the same demand.
What is tank autonomy?
Tank autonomy is the number of days your stored fresh water will last without any watermaker production. It is calculated by dividing your total tank capacity by your daily demand. A higher autonomy means more days of reserves if the watermaker fails or is shut down.
$$Tank\;Autonomy\;(days) = \frac{Tank\;Capacity} {Daily\;Demand}$$
Should I choose one large unit or two smaller units?
Many yacht designers recommend installing two smaller watermakers instead of one large unit. This provides redundancy: if one unit fails, the other can still produce fresh water. Two units also allow flexible operation — run one during light demand and both during peak periods. However, dual installations cost more and require additional space.
What maintenance does a watermaker require?
Marine watermakers require regular membrane flushing with fresh water (typically every 24–72 hours when not in use), periodic membrane cleaning or replacement, pre-filter changes, and pump service. Neglecting maintenance can reduce output capacity and shorten membrane life significantly.