How to read a fertiliser bag and get the rate right

You've got a bag of fertiliser, a hungry-looking lawn, and a label covered in numbers that may as well be written in another language. Don't worry — reading a fertiliser bag is genuinely simple once someone shows you the three things that actually matter. In this post we'll decode the NPK numbers, explain that "covers X square metres" claim (and why it's a bit optimistic), and walk through the easy maths to work out exactly how much you need. By the end you'll know how to set your spreader, avoid burning your grass, and water it in properly.

The three numbers on the front: NPK

Every fertiliser bag has three big numbers, like 20-5-10 or 15-0-15. That's the NPK ratio, and it tells you the percentage by weight of three nutrients:

• N — Nitrogen. Drives green, leafy top growth. This is the number that matters most for a lush lawn.
• P — Phosphorus. Helps roots and establishment. Often low or zero in lawn feeds (and restricted in some regions).
• K — Potassium. Toughens the plant against heat, cold, drought and disease.

So a 20-5-10 bag is 20% nitrogen, 5% phosphorus and 10% potassium by weight. The remaining 65% is filler, carrier granules and other minor nutrients.

The key insight: the NPK numbers are percentages, not amounts. A higher first number means more nitrogen per kilo of product — so you spread less of a strong feed to deliver the same amount of nitrogen as a weak one.

That single fact is what links the front of the bag to how much you actually apply. Hold onto it.

The "covers X sqm / sq ft" claim

Somewhere on the bag you'll see something like "covers up to 400 sqm" or "treats 4,300 sq ft". Treat this as a marketing ceiling, not a recommendation.

• It's usually the lightest possible rate. "Up to" coverage assumes you spread thinly. Feeding at that rate often gives weaker results than the bag implies.
• It ignores your lawn's actual condition. A struggling lawn in spring wants more than a healthy one in autumn.
• The number you should trust is the application rate, usually printed as grams per square metre (g/m²) or pounds per 1,000 sq ft. That's the real instruction.

If the bag gives you a rate, use the rate. The coverage figure is just there to help you guess how many bags to buy.

Bag rate vs nitrogen per area — the bit everyone confuses

Here's where people trip up. There are two different "rates" and they are not the same thing:

• Bag rate (product rate): how many grams of the whole product to spread per square metre. This is what your spreader delivers.
• Nitrogen rate: how much actual nitrogen lands per area. The agronomy-standard target is roughly 0.5 kg of nitrogen per 100 m² per feed (about 1 lb of N per 1,000 sq ft).

Because the bag is only partly nitrogen, you have to convert between the two. The good news is it's one short sum.

The one formula you need

To find the product rate that delivers a target amount of nitrogen:

Product needed = target nitrogen ÷ (N% ÷ 100)

That's it. Divide your nitrogen goal by the nitrogen fraction, and you get how much product to spread.

A worked example (the easy maths)

Say your lawn is 150 m² (about 1,600 sq ft) and you've bought a 20-5-10 fertiliser. You want the standard 0.5 kg of N per 100 m².

Step 1 — nitrogen you want in total: 150 m² ÷ 100 × 0.5 kg = 0.75 kg of nitrogen

Step 2 — product needed (20% N = 0.20): 0.75 kg ÷ 0.20 = 3.75 kg of product

Step 3 — product rate per m²: 3.75 kg ÷ 150 m² = 25 g/m²

So you'd spread 3.75 kg total, at 25 g/m². Here's how the same lawn changes with a stronger or weaker feed:

Same lawn, same nitrogen delivered — but the stronger the bag, the less you spread. That's the NPK insight in action.

Setting your spreader

Once you know your rate, you need to get it onto the lawn evenly.

• Check the bag for a spreader setting. Many list a number for popular broadcast and drop spreaders. It's a starting point, not gospel.
• Calibrate if you can. Weigh out the product for a small measured area, set the spreader low, and do a test pass. Adjust until the dial matches your target rate.
• Spread in two half-rate passes at right angles (e.g. north–south, then east–west). This evens out any striping far better than one heavy pass.
• Close the hopper on turns and keep your walking pace steady — speed changes the amount that drops.

Granular vs liquid

• Granular is the easiest for beginners: slow-release options feed for weeks, it's forgiving, and a spreader gives even coverage. Best for routine feeding.
• Liquid (hose-end or sprayer) acts fast and is great for a quick green-up or spoon-feeding small areas, but it leaves the system sooner and needs more frequent application.

For most home lawns, a granular slow-release feed is the low-effort, low-risk choice.

Avoiding fertiliser burn

Fertiliser burn is what happens when you apply too much — the salts pull moisture out of the grass and you get yellow or brown scorch marks, often in spreader-shaped stripes.

• Never double back over the same strip unless you've halved the rate.
• Don't eyeball it "to be safe." More is not better. Stick to the rate you calculated.
• Avoid feeding a bone-dry, heat-stressed lawn. Wait for cooler conditions or water first.
• Sweep granules off paths and beds so they don't wash into a concentrated puddle (or into drains).

If you do spill or over-apply in a spot, flush it with lots of water as soon as possible to dilute the salts.

Watering in

Granular fertiliser needs water to start working — it dissolves and moves down to the roots.

• Water within 24 hours of applying granular feed, with about 5–10 mm (roughly ¼ inch) of water.
• Don't water immediately after a liquid feed — let it sit on the leaf for a few hours first so it can absorb, unless the label says otherwise.
• Watering in also protects against burn, because it gets the salts off the leaf blade and into the soil.

A light watering is all you need — you're activating the feed, not flooding the lawn.

Frequently asked questions

What if my bag only shows "covers X sqm" and no rate?

Divide the bag weight by the coverage area to get a rough g/m², then sanity-check it against the 0.5 kg N per 100 m² guide using the N% on the front. If it's way under, lean toward the higher end of the coverage range.

Can I just use half a bag and guess?

You can, but you risk uneven feeding and burn patches. Weighing your product and doing two light passes takes five extra minutes and gives far better results.

How often should I feed?

Most lawns do well with a feed every 6–8 weeks during their growing season, easing off in dormancy. Slow-release granular products stretch that interval out.

Is a higher first number (more nitrogen) always better?

No — it just means you spread less product. A high-N feed pushes fast top growth, which can mean more mowing and more stress in heat. Match the feed to the season and your goals.

Do I really need to water it in?

For granular, yes — dry granules sitting on the leaf can scorch and won't feed the roots until they dissolve. A quarter-inch of water does the job.

How Lawnova takes the maths off your plate

Reading a bag is one thing; remembering to do it at the right rate, in the right season, for your grass is another. Lawnova builds you a personalised plan based on your lawn size, grass type and region — so it tells you exactly how much to apply, when to feed, and when to water it in, with the calculations already done. No spreadsheets, no guesswork, just the right amount at the right time.

Get your free Lawnova plan

Feed it right, and your lawn will thank you with the kind of green that makes the neighbours ask.