Tank Specific Gravity Ratings and How to Pick the Right One

Manufacturers reach a higher rating mainly through thicker walls and a stronger overall design. They also add plastic where the sidewall meets the bottom, because that's where the pressure is greatest.

Specific Gravity of Common Liquids

The liquids people store in tanks range from lighter than water to almost twice as heavy. These published figures run from lightest to heaviest, and the pounds per gallon are the specific gravity times 8.34, rounded.

The last column shows the first of our ratings (1.0, 1.5, 1.7, 1.9 or 2.0) that covers each liquid's weight. Where a liquid sits just under that rating, the column also gives the next rating up, which is the one to buy. The column covers weight only, not whether a tank's plastic can handle the chemical.

The strength of a chemical changes its specific gravity, sometimes enough to change the tank you need. Sodium hydroxide, also called caustic soda, is a good example. A 20% solution has a specific gravity of 1.23, a 25% solution is 1.28 and a 50% solution is 1.54. A 1.5 tank is built for liquids up to 1.5, so it covers the 20% solution's weight but not the 50% solution's. When you look up your liquid, use the figure for the exact strength you're buying.

Gasoline and diesel are lighter than water, so every rating covers their weight. Whether a tank can hold fuel depends on whether it's made for fuel storage and on your state and local fuel rules.

How to Find the Specific Gravity of Your Liquid

The quickest place to find your liquid's specific gravity is its safety data sheet (SDS). Section 9 of an SDS covers physical and chemical properties, and one of the properties it lists is "density and/or relative density." Relative density is another name for specific gravity. If the sheet gives density in grams per milliliter or kilograms per liter, you can read that number as the specific gravity, because water weighs about one kilogram per liter. If it gives pounds per gallon, divide by 8.34.

If you don't have an SDS, you can get the number other ways:

• Ask your supplier. The company selling you the fertilizer or chemical can give you the specific gravity for the exact product and strength you're buying.
  
  
• Use a hydrometer. A hydrometer floats in a sample of the liquid, and you read the specific gravity off its stem.
  
  
• Weigh a gallon. Weigh an empty container, fill it with exactly one gallon of the liquid and weigh it again. Divide the difference by 8.34. If the gallon of liquid weighs 11.1 pounds, its specific gravity is about 1.33. Wear whatever protective gear the product calls for while you handle it.
  
  

What About Sludge, Slurry and Other Liquids With Solids?

Sludge, slurry, manure and other mixes with solids in them don't have one published specific gravity, because the number depends on how much solid material is in the mix. Weighing a gallon of your actual mix is the best way to get a number. If the mix changes from load to load, weigh a few samples and plan around the heaviest one.

How to Pick the Right Specific Gravity Rating

1. Get your liquid's specific gravity at the strength you'll store. The table of common liquids, the SDS, your supplier or a weighed gallon will give it to you.
   
   
2. Buy the first rating above that number. That's the standard rule of thumb for poly tanks. It applies to every liquid except water: tanks rated 1.0 are built for water, so water can go in a tank rated exactly 1.0. A liquid at 1.33 goes in a 1.5 tank. A liquid at 1.54 goes in a 1.7 tank.
   
   
3. Step up one more rating only if your liquid sits close to the rating from step 2 or will be stored warm. A liquid is close when it sits within about 0.1 of that rating, like KTS fertilizer at 1.46 against a 1.5 tank, or 96 to 98% sulfuric acid at 1.84 against a 1.9. Published figures for the same product differ a little: 10-34-0, for example, is listed anywhere from 1.37 to 1.40. The batch you get can be heavier than the number you looked up. If your liquid already sits just under its rating, a heavier batch can push it over. Stepping up also makes sense for a liquid stored warm, because heat lowers how much weight a tank can hold.
   
   
4. Check chemical compatibility. Chemical compatibility is whether the tank's plastic holds up to your liquid. The rating doesn't tell you that. Tanks with the same rating can be made from different plastics: our 1.9-rated Peabody Gemini tanks come in both high-density polyethylene (HDPE) and cross-linked polyethylene (XLPE). Check the product listing and the manufacturer's chemical resistance information to see whether the plastic suits your liquid. If neither one says, ask us.
   
   

Does Temperature Change a Tank's Specific Gravity Rating?

A tank's specific gravity rating goes down as the liquid inside it warms up, because heat softens the tank wall. The rating on a Peabody Engineering tank, for example, assumes the liquid is at 73°F, so the rating starts dropping once the liquid is warmer than that. Peabody gives a second figure: at about 120°F, polyethylene tank walls start losing strength. If your liquid will get anywhere near that, the tank's temperature limit is the number to check before you order.

So when I say a liquid will be stored warm, I mean it will sit well above 73°F for long stretches. That's when to step up a rating.

What Can a 1.5 or 1.9 Specific Gravity Tank Hold?

A 1.5 specific gravity tank is built for liquids up to 1.5 times as heavy as water, about 12.5 pounds per gallon. By weight, that covers DEF, 12.5% sodium hypochlorite and the lighter liquid fertilizers. Our 1.5-rated tanks are in our 1.5 specific gravity collection.

A 1.9 specific gravity tank is built for liquids up to 1.9 times as heavy as water, about 15.8 pounds per gallon. By weight, that covers 50% caustic soda and 75% and 85% phosphoric acid. Our 1.9-rated tanks are from Peabody Engineering.

Can I Put Fertilizer or Chemicals in a Water Tank?

Our advice is to keep fertilizer and chemicals out of water tanks and put them in a fertilizer, chemical or industrial tank instead. If you're looking at a water tank anyway, check its rating first. Some poly tanks are rated only 1.0. The water tanks in our catalog are typically rated around 1.5. Many chemicals weigh more than 1.5, which rules out a water tank on weight alone.

For fertilizer, Purdue Extension recommends a tank rated at least 1.5, so a water tank rated around 1.5 just meets that minimum. A 1.5 tank is built to hold the weight of the lighter blends, like UAN 28 and UAN 32. Heavier blends like 10-34-0 and KTS sit close to 1.5, so they call for a 1.7 or higher. So does fertilizer that will be stored warm, because the rating drops as the liquid in the tank warms.

You'll find fertilizer and chemical tanks in our fertilizer tanks and chemical storage tanks collections. Tanks in those collections carry different ratings, so check the rating on each listing against your liquid's number. If your blend calls for more than 1.5, look at our 2.0-rated tanks.

Is a Specific Gravity Rating the Same as a Pressure Rating?

A specific gravity rating is not a pressure rating. It covers the push of the liquid's own weight on the tank wall, the push that grows with depth. The poly tanks these ratings appear on are built to hold liquid at normal air pressure, not under added pressure the way a pressurized tank does. Our guide to choosing a water storage tank explains how a storage tank differs from a pressure tank.

How Much Will the Full Tank Weigh?

A full tank weighs its capacity in gallons times 8.34 times the liquid's specific gravity, plus the weight of the empty tank. Take a 500-gallon tank. Full of water, it holds 4,170 pounds of liquid. Filled with a liquid whose specific gravity is 1.5, it holds 6,255 pounds. With a liquid at 1.9, it holds 7,923 pounds, almost four tons. The empty tank adds little by comparison: a 500-gallon Custom Roto-Molding vertical tank weighs 140 pounds.

All of that weight sits on the pad, stand or ground under the tank. Any tank needs a smooth, level base that supports its whole bottom. For tanks over 2,000 gallons, Peabody Engineering recommends a concrete pad built to carry the full tank.

Should I Just Buy the Highest Rating to Be Safe?

A higher rating than you need won't hurt, but you'll pay for it. Higher-rated tanks carry more plastic in the wall, so they weigh more and cost more. On Peabody Engineering's 62-gallon Gemini Flat Top Round tank, the 1.9-rated version costs $78 more than the 1.5-rated version ($877.99 against $799.99 in October 2026). It also weighs 5 pounds more empty. The gap is bigger or smaller depending on the model.

If your liquid sits close to a rating, will be stored warm or doesn't have a published number, give us a call at 833-465-8265 or email info@tanksupplydirect.com, both listed on our contact page. Tell us what's going in the tank and how much of it you need to store, and we'll help you match the rating, the material and the size before you order.