Ocean water contains about 35 grams of salt per liter, which works out to roughly 3.5% salt by weight. To recreate it at home, you need about half a cup of salt per gallon of water, but the type of salt and how you mix it matter more than most people expect. Your approach depends on what you need the salt water for: a saltwater aquarium, a science experiment, a therapeutic soak, or something else entirely.
The Basic Ratio
Average ocean salinity is 35 parts per thousand. In practical kitchen terms, that means dissolving about 35 grams (roughly 2 tablespoons) of salt into every liter of water, or about half a cup (130 grams) per gallon. This gets you to the 3.5% concentration found across most of the open ocean. Real ocean salinity ranges from about 32 to 37 parts per thousand depending on location, so you have some wiggle room.
For a quick science project or a saltwater gargle, regular non-iodized table salt or sea salt dissolved in warm water works fine. Stir until the salt fully dissolves, taste it (it should be noticeably salty but not overwhelmingly so), and you’re done. But if you’re making salt water for a marine aquarium or anything where living organisms are involved, you need a more precise approach.
Which Salt to Use
Real seawater isn’t just sodium chloride. It contains magnesium, calcium, sulfate, potassium, and dozens of trace minerals. Plain table salt only provides sodium and chloride, which makes it fine for a science demo but completely wrong for keeping marine fish or coral alive.
For aquariums, use a commercial marine salt mix. These blends are engineered to replicate the full mineral profile of natural seawater. Brands like Instant Ocean, Red Sea, and Tropic Marin are widely available at pet stores. They come with their own dosing instructions, typically a half cup per gallon, but always follow the specific product’s label since formulations vary slightly.
For therapeutic uses like a saltwater soak or nasal rinse, pure sea salt (without anti-caking agents) gives you a closer approximation to ocean chemistry than iodized table salt. Keep in mind that medical saline solutions use a much lower concentration: 0.9% for isotonic saline, compared to the ocean’s 3.5%. A full-strength ocean replica would be far too harsh for wound care or sinus rinsing.
Water Quality Matters
Tap water contains chlorine, chloramine, phosphates, and trace metals that won’t bother you in a glass of drinking water but can cause serious problems in an aquarium. Long-term use of tap water in a marine tank contributes to nuisance algae growth and can harm corals. If you’re mixing salt water for fish or invertebrates, purified water (reverse osmosis or deionized, often labeled RO/DI) is the standard. You can buy RO/DI water at most fish stores or install a home filtration unit.
For non-aquarium purposes, tap water is perfectly fine. Just let it sit for a few minutes after mixing to allow the salt to dissolve completely.
How to Mix It Properly
If you’re making ocean water for an aquarium, don’t just stir and pour. The salt needs time to fully dissolve and reach chemical equilibrium. Best practice is to mix your salt into the water using a small powerhead or pump for aeration, then let it circulate for 8 to 24 hours before use. Testing by Bulk Reef Supply found that most commercial salt mixes don’t dissolve fully clear within the first hour. Some brands took over 21 hours to become completely clear, and a few never fully cleared even after 24 hours.
Mix in a clean, food-safe container (a plastic bucket or storage bin works well). Add the water first, then gradually pour in the salt while the pump is running. Adding all the salt at once can cause it to clump at the bottom.
Checking Your Salinity
Eyeballing the amount of salt you add is fine for a one-time project, but for anything ongoing, you need a way to measure salinity. Two common tools are hydrometers and refractometers.
A hydrometer is a floating device that measures specific gravity, the density of your salt water compared to pure water. Ocean water at 25°C (77°F) has a specific gravity of roughly 1.023 to 1.025, which corresponds to about 34 to 37 parts per thousand salinity. Hydrometers are cheap (under $10) but less accurate and prone to giving misleading readings if air bubbles stick to them.
A refractometer is the more reliable option. You place a drop of water on a glass lens, look through the eyepiece, and read the salinity directly off a scale. Refractometers are more accurate, more durable, and only cost $20 to $40. For anyone maintaining an aquarium, a refractometer is worth the upgrade.
Temperature Affects Your Reading
Water temperature changes density, which means the same salinity will produce different specific gravity readings at different temperatures. A sample at 5°C can read 8 to 10 points higher on a specific gravity scale than an identical sample at 25°C. If you’re using a hydrometer, always check the water temperature and use the correction chart that comes with the device. Most hydrometers are calibrated for a specific temperature, often 77°F (25°C). If your water is significantly warmer or cooler, your reading will be off.
Refractometers are less sensitive to temperature variation but should still be calibrated with RO/DI water (which should read zero) before each use.
Quick Reference by Purpose
- Science project or demonstration: 35 grams of table salt per liter of tap water. Stir until dissolved.
- Saltwater aquarium: Commercial marine salt mix with RO/DI water, mixed with a pump for 8 to 24 hours, verified with a refractometer to a specific gravity of 1.023 to 1.025.
- Therapeutic soak or float tank: Pure sea salt in warm tap water at 35 grams per liter for ocean-strength, or adjust to comfort.
- Nasal rinse or wound care: Use a much weaker solution, about 9 grams per liter (0.9%), which matches your body’s natural salt concentration. Full ocean strength is too concentrated for these uses.
The ocean’s salt concentration is one of those numbers that sounds simple but means different things depending on what you’re trying to do. For a school project, two tablespoons per liter and a spoon will get you there in under a minute. For a reef tank, you’re looking at purified water, a precision salt blend, a circulation pump, and a day of patience before the water is ready.