A fertiliser label says 50 millilitres per square metre. A weedkiller says 5 litres per hectare. An American guide says 2 gallons per thousand square feet. They are all rates, and none of them can be compared with the others as printed. This converts whatever unit your label uses into litres per square metre, works out how much product your area actually needs, and then puts the container sizes side by side — because the rate tells you how much to apply, but the container size decides how much you buy, and those two numbers are almost never the same.
Rate, area and container
Labels quote coverage in whatever unit suits the product. This puts them in one column.
Just the number. The unit is the next control.
The area the product is actually going on, not the whole property.
Result
Why coverage rates are written in so many different units
A rate is a thickness waiting to happen. Fifty millilitres per square metre is fifty thousandths of a litre on each square metre, which is the same as fifty thousandths of a millimetre of depth — in other words, a depth. Every coverage rate is secretly a depth, and the reason labels disagree about how to write it is that each product's natural dose sits in a different range. Concentrated products are applied in millilitres; diluted ones in litres; agricultural ones over hectares. The unit is chosen to keep the number between about one and a hundred, which is comfortable to read and easy to measure out.
That convenience is exactly why the numbers cannot be compared directly. Five litres per hectare and five litres per hundred square metres differ by a factor of a hundred, and the only way to see that is to convert both to the same base. Litres per square metre is the natural base here, because it converts to a depth in millimetres by moving the decimal point three places — one litre per square metre is one millimetre, exactly.
Reading the label correctly is half the job
The most common error is not the arithmetic, it is the label. Products are sold as concentrates with a dilution ratio, and the stated rate usually refers to the diluted solution rather than the concentrate in the bottle. A label that says apply at 100 mL per square metre and dilute 1:100 means you apply a hundred millilitres of mixed solution, of which one millilitre is concentrate. Working out how much concentrate to buy means multiplying by the area and then dividing by the dilution.
The second error is the area. Rates are quoted per unit of treated area, and the treated area is rarely the whole garden. A lawn feed goes on the grass, not on the beds inside it; a border feed goes on the border. Measuring the wrong area is a far bigger source of error than any rounding in the conversion, and the lawn area figure and garden area figure both let you subtract what is not being treated.
The third is that some rates are per application and some are per season. A label giving a seasonal maximum is not telling you to apply it all at once; it is telling you the ceiling. Exceeding a seasonal maximum is a real problem with nitrogen and with some herbicides, and the rate alone will not warn you.
| Unit | To litres per m² | Typical use | Watch out for |
|---|---|---|---|
| mL per m² | divide by 1,000 | Concentrates, foliar feeds | Usually the diluted solution |
| L per m² | as written | Diluted feeds, watering | Also equals mm of depth |
| L per 100 m² | divide by 100 | Lawn and border products | Hundred, not thousand |
| L per hectare | divide by 10,000 | Agricultural labels | Looks enormous, is often small |
| US gal per 1000 ft² | multiply by 0.0407 | American guidance | US gallon, not imperial |
| fl oz per 1000 ft² | multiply by 0.000305 | American concentrates | Fluid ounces |
The container problem
Here is the part that catches people. The rate tells you how much to apply. The shelf tells you what you can buy, and those two numbers never line up. Twelve litres needed arrives as a twenty-litre drum or as twelve one-litre bottles, and the difference is not just convenience. Price per litre falls sharply with size — a concentrate that costs twelve dollars a litre in a bottle might cost four in a drum — so the cheapest way to buy twelve litres is very often to buy twenty.
That leaves eight litres over. Whether that matters depends entirely on shelf life. A diluted solution may not keep; a concentrate in a sealed drum usually keeps for years if stored frost-free and out of the sun. If it keeps, the twenty-litre drum is the obvious buy, because the leftover is not waste, it is next year's supply at a better price. If it does not keep, the leftover is genuinely wasted, and the small bottle wins.
A concentrate stored sealed, cool and frost-free typically keeps for several seasons. Check the label for a shelf life rather than assuming.
Once diluted, most products have a working life measured in hours or days, not years. Mix only what you will apply.
A worked example
- Convert the rateFifty millilitres per square metre is 0.05 litres per square metre — divide by a thousand.
- Apply the areaTwo hundred and forty square metres at 0.05 L/m² is 12 litres of product.
- Look at the sizesTwelve litres is twelve bottles, five jugs of 2.5 L, three drums of 5 L, two of 10 L or one of 20 L.
- Price themAt typical shelf prices that is $144 in bottles, $112.50 in jugs, $97.50 in 5 L drums, $100 in 10 L drums and $80 in a single 20 L drum.
- Decide with the leftover in viewThe drum is cheapest but leaves eight litres over. The bottles leave nothing over and cost almost twice as much.
The effective price is the number to look at: eighty dollars for twelve litres actually used is $6.67 per litre used, against twelve dollars for the bottles. Even after writing off eight litres of the drum, the drum wins comfortably — and it wins by more if the concentrate keeps.
Measuring liquid product accurately
Converting correctly is wasted effort if the measuring is rough. Use a dedicated measuring jug kept for the purpose, rinse it into the tank rather than into the ground, and measure at eye level on a flat surface. Kitchen spoons are not measuring equipment: a tablespoon varies between ten and twenty millilitres depending on the spoon and on how generously it is filled.
For very small rates, the reliable method is to make a stock solution: measure a larger, easily measured quantity into a known volume of water, then apply part of that. Measuring five millilitres directly is inaccurate; measuring fifty millilitres into five litres and applying a tenth of the result is not.
Calibrating the sprayer once is worth more than any of this. Mark the tank at a known volume, time how long it takes to empty over a measured area, and you know your output per minute — which turns every future application into a matter of walking speed rather than guesswork.
Do
- Convert to litres per square metre before comparing two products.
- Check whether the stated rate is concentrate or diluted solution.
- Measure the area that is actually being treated, not the whole plot.
- Compare container sizes, because price per litre falls sharply with size.
- Check the shelf life before deciding whether leftover is waste or stock.
Avoid
- Comparing a rate per hectare with a rate per hundred square metres as printed.
- Assuming a US gallon and an imperial gallon are the same size.
- Applying a seasonal maximum in a single application.
- Measuring small volumes with kitchen spoons.
- Storing diluted product and expecting it to work next season.
Where the numbers stop being true
The container prices are typical retail figures and they vary by product, by brand and by country. The shape of the relationship — price per litre falling with size — is reliable; the exact figures are indicative. Check the shelf before committing to a size.
No allowance is made for the area being uneven. A rate per square metre of plan area applied to sloping or heavily textured ground delivers more product per unit of horizontal area than intended, and on steep ground the run-off means less reaches where you wanted it.
Nothing here accounts for overlap. Walking a sprayer over the same strip twice doubles the local rate, and double application is the most common cause of scorch. Where the water side matters, the irrigation run time figure handles the same depth-to-volume conversion in the opposite direction, and the fertiliser figure does the same job where the rate is given by nutrient rather than by product.
How the figures are produced
The rate you enter is multiplied by the conversion factor for the unit you selected to give litres per square metre. Multiplying by the area gives the volume of product. Each container size is then divided into that volume and rounded up to whole containers, and the volume actually bought is the container count times the size. The leftover is bought minus needed, and the cost is bought multiplied by a typical price per litre for that size. The cheapest row is the lowest cost; the least-waste row is the lowest leftover, and they are highlighted separately because they are usually different rows.
Questions people ask
Is litres per square metre the same as millimetres?
Yes, exactly. One litre over one square metre is a depth of one millimetre. That is why coverage rates and irrigation depths are the same quantity wearing different clothes.
My label says dilute 1:100. Does the rate apply to the concentrate?
Almost always the rate refers to the diluted solution. Apply at the stated rate of solution, and work out the concentrate separately by dividing by the dilution ratio.
Why is the cheapest size the one that wastes the most?
Because price per litre falls faster than the leftover grows. Twenty litres at four dollars is eighty dollars for twelve litres used; twelve bottles at twelve dollars is a hundred and forty-four for the same twelve litres, with nothing left.
How long does leftover concentrate keep?
Check the label, but sealed, cool and frost-free, most concentrates keep for several seasons. Emulsions and suspensions separate over time and may not re-mix properly.
Can I use an American rate?
Convert it, and check whether the gallon is US or imperial. A US gallon is 3.79 litres; an imperial gallon is 4.55. Confusing them changes the dose by twenty percent.
What if my sprayer output is unknown?
Calibrate it: spray a measured area, note the volume used, and you have litres per square metre delivered. Without that, the rate on the label cannot be turned into a walking speed.