Frying oil cost: cost per batch and when to change it

Frying oil costs you what you pay to fill the fryer plus what you top up each day, divided by every portion you fry before you change it. Three numbers from your own notebook (litres in the fill, litres topped up, portions sold) give you the cost per portion. When to change the oil is your call, made by looking at the oil and the food; the arithmetic tells you what each extra or fewer day is worth in dollars.

The three numbers you need

"About a bottle a day" is not a cost. You need:

1. Litres in the fill. How much oil the fryer or pot takes up to the working line. If you have two fryers, count each one separately.
2. Litres topped up. Food carries oil out with it. The level drops every day and you bring it back up. Those litres are bought too.
3. Portions per cycle. How many portions you fry from fresh oil until you throw it out. A cycle might be three days or a week, depending on your kitchen.

Then:

oil per portion = (fill litres + top-up litres in the cycle) × price per litre ÷ portions in the cycle

The price per litre comes from your receipt. If you buy 20-litre containers, divide the price by 20. To compare a large container against bottles, the unit price tool shows which one is cheaper per litre.

How to measure what the food takes

Put a strip of tape or a marker line at the working level. At closing, once the oil has cooled, top up with a measuring jug back to the line and write down the litres. Divide by the portions you fried that day.

If you top up 2 litres after 80 portions, each portion took 25 ml. That number is yours: it holds for that food, that cut and that fryer. Thick-cut fries, breaded chicken and battered fish do not behave the same, so if you fry different things in different fryers, measure each one. If everything goes into one fryer, measure the day's total and spread it over all portions.

The most common mistake is counting only the fill. The top-ups in a cycle can add up to more than half of what the fill cost, and if they stay out of the cost, the dish looks cheaper than it is.

When to change the oil: signs and records

No number of days fits every kitchen. It depends on what you fry, how hot you run it, whether you filter and how many crumbs fall in. What works is watching and writing it down. Practical signs you can check each day:

• The oil is much darker than at the start of the cycle.
• It smokes sooner than it did when fresh.
• It foams and the foam does not settle.
• Food browns on the outside before it is cooked through.
• Food picks up a smell or taste that is not its own.
• The oil feels thicker.

Keep a simple log: date of the fill, litres, each day's top-up, each day's portions, and what you noticed on the day you changed it. After a few cycles you will know how many days you get at the quality you want. If you try filtering at closing, or moving fish to its own fryer, the same log tells you whether the oil lasts longer and what that saves.

If you are unsure about local rules on reusing or disposing of used oil, check with your local authority.

Worked example: fries at a burger shop

You run a small burger shop. These are your numbers, from your own log:

• Fryer holds 15 litres to the line.
• You buy vegetable oil in 20-litre containers at $40.00, so $2.00 a litre.
• You sell 80 portions of fries a day.
• You change the oil every 5 days.
• You top up 2 litres every night.

Cost of one 5-day cycle:

• Fill: 15 × $2.00 = $30.00
• Top-ups: 2 × 5 = 10 litres; 10 × $2.00 = $20.00
• Cycle total: 30.00 + 20.00 = $50.00
• Portions in the cycle: 80 × 5 = 400
• Oil per portion: 50.00 ÷ 400 = $0.125

If you had counted only the fill, you would get 30.00 ÷ 400 = $0.075 a portion, $0.05 short. Open 26 days a month, you sell 80 × 26 = 2,080 portions, and 2,080 × 0.05 = $104 a month of oil that was missing from your cost.

What happens if you change sooner or later, with the same daily use:

Each extra day lowers the cost per portion, but by less each time: from 4 to 5 days you save about $0.019 a portion; from 5 to 6, $0.0125. Going from 5 to 6 days would save 2,080 × 0.0125 = $26 a month. That is the price of the decision. If the fries come out dark or taste of old oil on day six, $26 does not cover the regular who stops coming. The table does not decide for you; it tells you what the decision you make by looking at the fryer is worth in money.

What to add and what to keep separate

• Add: if you pay someone to collect your used oil, that fee belongs to the cycle. Divide it by the same portions.
• Add: oil thrown away mid-cycle after a spill or a deep clean. Log it in the cycle where it happened.
• Keep separate: the gas or electricity the fryer uses. That is a kitchen cost, spread as explained in how to allocate overhead to products.
• Keep separate: oil for sautéing or dressing. That goes into each recipe in millilitres.

Where it lands in your dish

Oil per portion is one more line in the dish cost, next to the potatoes, the sauce and the box. To see the full cost of something fried, look at what fried chicken costs to make, where the oil sits alongside the chicken, the breading and the waste. And to see oil next to other ingredients by type, the guide to ingredient costs by category puts them together.

Do it with the calculator

In the food cost calculator, add the oil as an ingredient in the recipe, using the millilitres per portion your cycle gives you. In the example that is 25 litres ÷ 400 portions = 62.5 ml per portion at $2.00 a litre. When the price of a container changes, you update that one number and the dish cost follows.