How to Size a Seawater Watermaker: GPD for Boats, Resorts and Island Sites

A seawater watermaker is sized in gallons per day (GPD), but the number on the label is a 24-hour rating at stated water conditions. Pick the unit from three things: how much water you use, how many hours a day the machine will run, and how cold the sea water gets. The math is below, matched to AMPAC models.

Step 1: Count your daily water use

Start with a number you can defend. If you have a meter or a tank log, use it. If you do not, a household is a reasonable starting point: the US EPA says the average American family uses more than 300 gallons of water per day at home, and roughly 70 percent of that is indoors (US EPA WaterSense, 2026). Island homes, guesthouses and small communities can scale from there.

• Vessels: count people on board, then add the galley, laundry and washdown. Use your own per-person figure from your tank log.
• Resorts and guesthouses: count guests and staff, then add laundry, kitchen and landscaping.
• Remote sites and camps: count the crew and the equipment that needs fresh water.

Step 2: Turn daily use into a rated GPD

The rating on a watermaker is the water it makes in 24 hours of running. If you only run it part of the day, the rating has to be higher. The formula is:

Required rating (GPD) = daily need x 24 / hours you will run it each day

Example (assumed numbers)	Daily need	Run hours	Required rating
One household, using the EPA family figure	300 gal	12	600 GPD
Small community of 20 homes at 300 gal each	6,000 gal	16	9,000 GPD
Vessel with 10 people at an assumed 25 gal each	250 gal	6	1,000 GPD

Replace the assumed figures with your own. The vessel row uses 25 gallons per person only as a placeholder, so swap in what your tank log shows.

Step 3: Allow for cold water and the feed conditions

Production ratings on AMPAC product pages are based on stated feed conditions. For example, the 1,000 GPD watermaker lists feed water below 45,000 ppm TDS, 30% recovery, 98% rejection and an average temperature of 25 degrees Celsius (77 degrees Fahrenheit). The land-based SW10K-LX lists feed water below 50,000 ppm TDS. If your sea water is colder than the rating, or saltier, plan for margin and tell us your lowest water temperature when you ask for a quote. How much temperature matters depends on the membrane. As one reference point, Dow’s FILMTEC manual says a feed temperature drop of 4 degrees Celsius causes a permeate flow decrease of about 10% (Dow FILMTEC Technical Manual, section 5.6.6).

Step 4: Match the rating to a model

Required rating	Models to look at
Up to 150 GPD (emergency)	150 GPD portable emergency watermaker
100 to 1,000 GPD	100, 200, 300, 600, 800 and 1,000 GPD RO watermakers
1,500 to 6,000 GPD	1,500, 2,000, 3,000, 4,500, 5,000 and 6,000 GPD RO watermakers
3,000 to 80,000 GPD (land based)	SW3000-LX, SW5000-LX, SW6000-LX, SW9000-LX, SW10K-LX, SW12000-LX, SW20K-LX, SW40K-LX, SW60K-LX, SW80K-LX
80,000 GPD and up (containerized)	SW80K-LXC, SW100K-LXC and the SW100K-LX mobile system
Offshore, Class 1 Division 2	SW10K-C1D2 (10,000 GPD) and SW20K Class 1 Division 2

Capacity on each page is listed in gallons and liters per day. The SW10K-LX page lists 10,000 GPD, 37,850 LPD and 1.6 cubic meters per hour. The full range is on the seawater desalination systems page.

Step 5: Check your feed water and plan for the brine

The USGS says ocean water contains about 35,000 ppm of salt. It classes water from 3,000 to 10,000 ppm as moderately saline and water from 10,000 to 35,000 ppm as highly saline (USGS, 2026). If your source reads at the low end, such as an estuary or a brackish well, a brackish water RO system may fit better than a seawater unit.

Every seawater system also makes brine. Plan where it goes before you buy. Our guides on recovery rate and reject water and boron removal cover the two questions that come up most, and the seawater desalination RO guide explains the process end to end.

Special cases

• Continuity matters: the twin redundant 2 x 2,000 GPD watermaker gives you a second unit if one is down for service.
• Classified areas: offshore platforms need the explosion-proof Class 1 Division 2 models above.
• Containerized sites: the mobile units come fully packaged as automated, turnkey systems.
• Power: product pages list the electrical options, such as 110V, 220V, 380V and 460V. Tell us what you have.

Frequently Asked Questions

Is a watermaker’s GPD rating per day or per hour?

Per day, based on 24 hours of running. Pages also list liters per day and cubic meters per hour, so you can check both. If you run the machine fewer hours, use the formula in step 2 to find the rating you need.

Why size a watermaker above my daily use?

Because you will not run it 24 hours a day, and because the rating assumes stated feed conditions such as 25 degrees Celsius water. Colder or saltier feed water lowers output, so a margin keeps you supplied.

Can I install a backup unit?

Yes. Some sites use two smaller units so one can be serviced while the other runs. AMPAC builds a twin redundant 2 x 2,000 GPD model, and the SW24000-LX puts two 12,000 GPD units on one skid.

What do you need to quote a seawater watermaker?

Your daily water use, run hours, sea water temperature and salinity if known, the power available, where the unit will be installed and where the brine will go. A vessel, platform or land site each has its own checks, so say which one it is.

Conclusion

Work out the daily need, convert it to a rated GPD with your run hours, add margin for cold water, then match it to a model. Request a quote from AMPAC Water Systems and our engineers will confirm the size against your site.

Sources

• US EPA WaterSense: How We Use Water
  The average American family uses more than 300 gallons of water per day at home, about 70 percent of it indoors.
• USGS Water Science School: Saline water and salinity
  Salinity classes in ppm and the figure of about 35,000 ppm of salt in ocean water.
• USGS Water Science School: Desalination
  How reverse osmosis desalinates saline water through a semi-permeable membrane.
• Dow FILMTEC Reverse Osmosis Membranes Technical Manual, Form No. 609-00071-0416 (mirror copy)
  Section 5.6.6: a feed temperature drop of 4 degrees Celsius causes a permeate flow decrease of about 10%. A 2016 manual, so check your membrane maker’s current data.