Resources
Specific Gravity and Tank SG Ratings
What specific gravity means, how to choose a tank’s SG rating, and a chart of common chemicals with their specific gravity and weight per gallon.
The basics
What Specific Gravity Means
Specific gravity (SG) compares how heavy a liquid is to the same volume of water. Water is 1.0. A liquid with an SG of 1.5 weighs 1.5 times as much as water, so a gallon of it weighs about 12.5 pounds instead of 8.34.
A tank’s SG rating
A tank’s SG rating is the heaviest liquid it is built to hold. Heavier liquids push harder on the tank wall, so a higher-rated tank is molded with a thicker wall. The ASTM D1998 standard bases wall thickness on the liquid’s specific gravity.
The rule of thumb
Choose a tank rated at or above the specific gravity of your liquid. A lighter liquid in a higher-rated tank is fine. A heavier liquid in a lower-rated tank is not.
Pounds per gallon
Multiply specific gravity by 8.34 to get pounds per gallon. To go the other way, divide pounds per gallon by 8.34.
Tank ratings
Our Tank SG Ratings
| Tank rating | Heaviest liquid | Built for |
|---|---|---|
| Water | SG 1.0 · 8.34 lb/gal | Water only |
| 1.5 SG | SG 1.5 · 12.5 lb/gal | Most water-based chemicals, many acids, caustics up to about 1.5 SG |
| 1.9 SG | SG 1.9 · 15.8 lb/gal | Heavy chemicals such as 50% caustic soda, phosphoric acid and concentrated sulfuric acid |
| 2.2 SG | SG 2.2 · 18.3 lb/gal | The heaviest liquids and slurries |
Chart
Specific Gravity of Common Chemicals
Typical values at room temperature, from producer data sheets. Your product’s SG depends on its exact concentration and temperature, so check the safety data sheet (SDS) from your supplier. Highlighted rows need a 1.9 SG tank.
| Chemical | Typical SG | Pounds per gallon | Minimum tank rating |
|---|---|---|---|
| Water | 1.00 | 8.3 | Water |
| Diesel exhaust fluid (DEF), 32.5% urea | 1.09 | 9.1 | 1.5 SG |
| Hydrogen peroxide, 35% | 1.13 | 9.4 | 1.5 SG |
| Hydrogen peroxide, 50% | 1.20 | 10.0 | 1.5 SG |
| Hydrochloric (muriatic) acid, 31–32% | 1.16 | 9.7 | 1.5 SG |
| Hydrochloric (muriatic) acid, 37% | 1.18–1.19 | 9.9 | 1.5 SG |
| Sodium hypochlorite (bleach), 12–15% | 1.16–1.21 | 9.7–10.1 | 1.5 SG |
| Hydrofluosilicic (fluorosilicic) acid, 23–25% | 1.19–1.22 | 9.9–10.2 | 1.5 SG |
| Sodium hydroxide (caustic soda), 25% | 1.27 | 10.6 | 1.5 SG |
| UAN 28 liquid fertilizer | 1.28 | 10.7 | 1.5 SG |
| UAN 32 liquid fertilizer | 1.32 | 11.0 | 1.5 SG |
| Aluminum sulfate (liquid alum), 48% | 1.29–1.35 | 10.8–11.3 | 1.5 SG |
| Nitric acid, 67–70% | 1.41 | 11.8 | 1.5 SG |
| Ferric chloride, 40% | 1.38–1.49 | 11.5–12.4 | 1.5 SG |
| Sodium hydroxide (caustic soda), 50% | 1.52–1.53 | 12.7 | 1.9 SG |
| Phosphoric acid, 75% | 1.57 | 13.1 | 1.9 SG |
| Phosphoric acid, 85% | 1.69 | 14.1 | 1.9 SG |
| Sulfuric acid, 93–98% | 1.83–1.84 | 15.3 | 1.9 SG |
Your liquid
How to Find Your Liquid’s Specific Gravity
- Check the SDS. Section 9 of the safety data sheet lists specific gravity or relative density.
- Ask your supplier for the certificate of analysis, which often lists the measured SG.
- Measure it with a hydrometer, or weigh a known volume: pounds per gallon ÷ 8.34 = SG.
- Allow for the range. Concentration and temperature change SG. Use the highest value your product can reach, and tell us your storage temperature, because plastic tanks lose strength as they get warmer.
Weight
How Much Will a Full Tank Weigh?
Gallons × SG × 8.34 = pounds of liquid. Plan the pad, stand or floor for the full weight plus the tank itself.
| Example | Liquid weight |
|---|---|
| 1,000 gallons of water (SG 1.0) | about 8,340 lb |
| 1,000 gallons of 12.5% bleach (SG 1.2) | about 10,000 lb |
| 1,000 gallons of 50% caustic soda (SG 1.52) | about 12,700 lb |
| 1,000 gallons of 93% sulfuric acid (SG 1.83) | about 15,300 lb |
FAQ
Specific Gravity Questions
Can I store a heavier liquid in a lower-rated tank?
No. A liquid heavier than the tank’s SG rating puts more stress on the wall than the tank was designed for. Choose a tank rated at or above your liquid’s specific gravity.
Can I store water in a 1.9 SG tank?
Yes. A lighter liquid in a higher-rated tank is fine. A higher rating also gives you room to switch to a heavier chemical later.
Which rating do I need for 50% caustic soda?
50% sodium hydroxide has a specific gravity of about 1.52–1.53, just over 1.5, so it needs a 1.9 SG tank. 25% caustic, at about 1.27, fits a 1.5 SG tank.
Does temperature matter?
Yes. Plastic tanks lose strength as they get warmer, and the ASTM D1998 standard reduces the allowable wall stress above 73.4°F. Tell us your storage temperature when you request a quote.
Where do I find my chemical’s specific gravity?
Section 9 of the safety data sheet (SDS) lists it, often as relative density. Your supplier’s certificate of analysis may also show the measured value.
Not Sure Which Rating You Need?
Send us your chemical, concentration, temperature and volume. We’ll recommend the tank and SG rating.