If you’ve ever looked at a tank spec sheet and seen “SG 1.5” or “SG 1.9” and wondered what it means, here’s the direct answer: SG stands for specific gravity. It tells you how dense a liquid the tank can safely hold, compared to water. Water sits at SG 1.0. Anything denser needs a tank built for a higher number. Get that number wrong, and you risk cracking the tank wall.
This post breaks down what the SG rating on an industrial poly tank actually means. We’ll cover why the number on the tank is usually higher than your liquid’s actual density, and how to check you’re buying the right one.
What does the SG number on a tank spec sheet actually mean?
Specific gravity is a simple ratio: how much a litre of your liquid weighs compared to a litre of water. Water is the baseline at SG 1.0. A liquid that’s 30% heavier than water sits at SG 1.3. A liquid twice as heavy as water is SG 2.0.
That number matters because tank walls carry weight, not volume. Manufacturers design a standard poly water tank to carry the load of water and nothing heavier. Fill it with a denser liquid, and you put more strain on the walls than the mould was ever built to handle. From the outside, though, the tank looks identical.
Common Poly Tank SG ratings and what they’re used for
Most Australian farms and businesses will run into a handful of standard ratings:
- SG 1.0–1.1 — Water and very light-density liquids. Standard rainwater and potable water tanks sit here.
- SG 1.3 — Common for liquid fertilisers and many agricultural chemicals, which typically run 1.3–1.5 in real-world density.
- SG 1.5 — A step up for heavier fertiliser blends and a wider range of industrial liquids.
- SG 1.9 — Used for denser acids, alkalis and industrial process liquids where extra wall strength is non-negotiable.
- SG 2.0 and above — Specialty applications, usually built to order.
The right number always comes back to the specific liquid, not the general category — two fertiliser blends can sit at different densities depending on concentration.
Why tanks are rated above the chemical’s actual SG
Here’s the part that trips people up: manufacturers rarely rate a tank at exactly the SG of the liquid it holds. A tank holding a liquid with an actual SG of 1.3 might carry an SG 1.5 or SG 1.9 rating on the plate.
That gap is a safety margin, not a mistake. It accounts for temperature swings, concentration changes, and splash loading during filling. You’ll also top up, drain, and refill a tank thousands of times over its 20-year working life. Buying a tank rated right on the edge of your liquid’s density leaves no room for any of that.
What happens if you use the wrong SG tank
Underrating a tank doesn’t fail immediately — it fails slowly. Walls bulge outward under sustained load, and stress lines appear around fittings and welds. Eventually the tank splits or collapses, usually without much warning. On a working farm or industrial site, that means a chemical spill, a lost batch of fertiliser, and potentially a safety incident — not just a warranty claim.
The fix is straightforward: match the tank’s SG rating to your liquid’s actual density, with margin, before you order — not after installation.
Static storage vs cartage tanks — does the SG rating change?
Yes. A cartage or transport tank carries the same liquid but adds motion. Braking, cornering, and road vibration all load the walls differently than a tank that sits still. Manufacturers often build cartage tanks to a higher SG spec than an equivalent static storage tank, precisely because the loading conditions are harsher. If you move liquid fertiliser or chemicals by road as well as store it on-site, check the SG rating separately for each tank in the system. The static tank spec doesn’t automatically carry over to the cartage tank.
How to check a tank’s SG rating before you buy
Three things to confirm before you order:
- Get the actual density of your liquid. Your Safety Data Sheet (SDS) will list it, or your supplier can confirm it.
- Ask for the tank’s rated SG, not just its “chemical tank” label. “Chemical-grade” on its own doesn’t tell you the number — ask for the specific rating.
- Confirm the rating covers your liquid with margin, not just at the exact density on the day of filling.
At The Water Tank Factory, we ask for your SDS before quoting any chemical or fertiliser storage application. That’s because the SG rating drives the tank spec, the fittings, and the wall thickness together, not separately. If you’re not sure what you’re dealing with, give us a call on 1300 826 532 and we’ll work it out with you. Or go to our contact form and leave us a message.
Match the Rating to the Liquid
A tank that’s underrated for its contents doesn’t usually fail on day one. It fails quietly, through months of temperature swings and refill cycles, until a wall gives way without much warning. That’s the real cost of guessing at an SG rating: not a wasted quote, but a spill, a shutdown, or a safety incident somewhere down the track. It’s also one of the cheapest checks in the entire buying process, and one of the few that protects everything else about the tank — the fittings, the wall thickness, the warranty. Get it right the first time, not the second.
Frequently Asked Questions
What does SG mean on a poly tank?
SG stands for specific gravity — a rating of how dense a liquid the tank can safely hold, measured against water at SG 1.0.
What SG rating do I need for liquid fertiliser?
Most liquid fertilisers sit between SG 1.3 and 1.5. The exact figure depends on the blend and concentration, so check your SDS before choosing a tank.
Can I use a water tank to store chemicals?
No. Standard water tanks handle SG 1.0 and will overload under a denser liquid, leading to bulging, cracking, and eventual failure.
Does a higher SG rating mean a thicker tank?
Generally yes — higher SG ratings require more wall thickness and reinforcement to carry the extra load safely. That’s part of why chemical-rated tanks cost more than standard water tanks. Wall thickness itself comes down to shot weight — read more on what shot weight means and why it matters.
Is the SG rating the same for storage and cartage tanks?
Not always. Cartage tanks face extra loading from road transport, so manufacturers often rate them higher than a static tank holding the same liquid.