Unit Price Shopping: How to Compare Groceries by Cost Per Serving

The cheapest package on the shelf is rarely the cheapest food on your plate. To compare groceries honestly, convert every option to one of two numbers: cost per 100 g (or 100 ml) for the raw ingredient, and cost per serving for the way you actually eat it. Unit price tells you which package is cheaper per gram. Cost per serving tells you whether that saving survives trimming, cooking, storage and waste. When the two disagree, cost per serving is the number that shows up in your bank account.

What unit price actually measures

Unit price is a rate, not a verdict. It answers one narrow question: how many dollars does one gram of this product cost in this package? That is genuinely useful, because package sizes are deliberately hard to compare. A 1.2 kg bag and a 600 g bag sit next to each other with different prices and different type sizes, and the bigger number is not automatically the better deal.

To get a unit price, divide the price by the amount:

• Price divided by total grams, then multiplied by 100, gives cost per 100 g.
• Price divided by total millilitres, then multiplied by 100, gives cost per 100 ml.
• Price divided by the number of servings in the package gives cost per listed serving.

Most large supermarkets already print this on the shelf label, usually in small type under the main price. Those labels are a useful shortcut, but they are not always complete: they often compare within one brand's range, they can be wrong after a price change, and they never account for what happens after you get the food home.

The three traps that flip the answer

Trap 1: drained weight

For anything packed in liquid, the number on the front is not the amount of food you get. A 400 g can of beans might contain 240 g of beans and 160 g of liquid. If you compare that can against a 400 g bag of dry beans, you are comparing two different products. Always compare the drained weight for canned goods, and remember that dry beans roughly triple in weight when cooked, so the honest comparison is cooked weight against cooked weight.

Trap 2: the package you will not finish

A 2 kg bag of flour at a lower unit price is only cheaper if you use 2 kg of flour before it goes stale or gets infested. If half of it ends up in the bin, your effective cost per usable gram is roughly double what the shelf label promised. This is where cost per serving beats unit price: it prices the food you actually consume, not the food you carry home.

Trap 3: preparation loss

Whole ingredients lose weight before they reach the plate. Chicken loses fat and moisture. Vegetables lose peels, stems and trimmings. Meat loses more if you trim it yourself. Pre-cut and pre-trimmed products charge you for that labour, and the premium is often much larger than people expect. A 10 to 20 percent yield difference is normal, and it is usually enough to change which option wins.

Worked example: chicken for four dinners

Say you are cooking four dinners for two people and each portion uses 160 g of cooked chicken. That is 640 g of cooked meat. Chicken yields roughly 75 percent of its raw weight after cooking, and you lose about 10 percent to trimming, so you need around 950 g of raw chicken to get there.

Here are three options from the same store:

• Family pack, 1.2 kg for $9.60. That is $0.80 per 100 g. After trimming you have about 1.08 kg usable. You use roughly 950 g for the four dinners, at a cost of about $6.82, and you still have about 350 g raw for a fifth meal.
• Standard pack, 600 g for $5.40. That is $0.90 per 100 g. You cannot cover 950 g with one pack, so you buy two for $10.80 and use 950 g of the 1.08 kg usable. The four dinners cost about $7.67.
• Pre-cut stir-fry strips, 400 g for $6.40. That is $1.60 per 100 g, and you lose very little to trimming, so call it 380 g usable. To reach 950 g raw you need two and a half packs, or $16.00 of chicken. The four dinners cost about $16.00.

Same meals, same protein, same week: the family pack costs $6.82, the standard pack $7.67, and the pre-cut strips $16.00. The gap between the family pack and the strips is more than $9 on one ingredient in one week. Add vegetables, rice, oil and aromatics, and the finished dinner lands at roughly $2.50 per serving on the family-pack version.

That is the whole method in one example. The package with the lowest headline price per gram was not the cheapest per meal, and the most convenient option was the most expensive by a wide margin.

How to do this in a real store in under two minutes

You do not need to calculate every item. Pick the eight to fifteen things you buy most often, and learn those numbers once.

1. Open your store's website or app before you shop and note the price and size of your staples.

2. Write the cost per 100 g next to each one. A phone calculator is enough.

3. Note the "buy price" you are willing to pay, and only stock up when the shelf price drops below it.

4. In the store, compare only the shortlist, not the whole aisle.

5. After cooking, write down how many servings the package actually produced.

That last step is the one people skip, and it is the one that makes the system accurate. After two or three weeks you will know that a certain bag of rice gives you eleven side servings and a certain bag of frozen peas gives you six, and you can plan a week without doing arithmetic in the aisle.

Where cost per serving changes the ranking

Cost per serving often reverses the unit-price winner, and it is worth knowing the cases in advance.

• Spices and seasonings. The cheapest per gram is usually a huge bag you will never finish. Small amounts from a bulk bin are often the better buy, even at a higher unit price.
• Fresh herbs. A bunch at a low unit price that wilts in three days costs more per usable gram than a smaller bunch you use completely.
• Frozen vegetables. They frequently beat fresh on cost per serving because there is almost no waste, and the nutrient content is comparable because freezing happens soon after harvest.
• Bread. A loaf with a lower unit price is only cheaper if you freeze the second half the day you buy it.
• Yoghurt and dairy. Large tubs beat single cups on unit price by a wide margin, but only if you portion them yourself and finish the tub.

Limitations: when unit price is the wrong question

This method has real limits, and pretending otherwise is how people end up with a pantry full of food they resent.

• It ignores quality and preference. The cheapest option you do not enjoy is not cheap, because you will buy something else instead.
• It ignores storage capacity. A bulk pack that does not fit in your freezer has an effective shelf life of zero.
• It ignores time. If pre-cut vegetables are the difference between cooking and ordering delivery, the convenience premium is worth paying.
• Sales and loss leaders move. A unit price learned three weeks ago can be wrong today, especially on meat, dairy and produce.
• Shelf labels are not always correct. They are maintained by hand in many stores and can lag behind price changes.
• It says nothing about nutrition. Cost per calorie and cost per gram of protein are different calculations, and they favour different foods.

Turning the numbers into a weekly list

The point of unit price work is a list you can reuse. Once you know your buy prices, build the week around what is cheap right now instead of around a recipe you found first. That is the order that keeps a grocery budget stable: prices first, meals second, recipes third.

If you want the list-building part handled for you, start with the weekly grocery list template and keep the arithmetic to a handful of staples. Then put the week together in the public planner, which turns a list of ingredients into actual meals instead of a pile of good intentions. For a one-person version of the same exercise, the cheap healthy grocery list for one person applies these ideas at a smaller scale.

FAQ

Is the lowest unit price always the best buy?

No. The lowest unit price wins only when you will use the whole package before it spoils, when preparation loss is similar between options, and when storage space is not a constraint. When any of those fail, cost per serving is the better guide.

Should I compare cost per 100 g or cost per serving?

Use cost per 100 g to compare two packages of the same ingredient. Use cost per serving to compare different forms of that ingredient, such as dry beans against canned beans, or whole chicken against pre-cut strips. The second number is the one that reflects your real spending.

Do store unit price labels already do this work?

Partly. They usually compare similar products within one store and one unit, which saves time. They do not account for drained weight, trimming loss, waste or storage, so treat them as a starting point rather than a final answer.

How many items should I track?

Eight to fifteen staples is enough to see a real change. Most households spend a large share of their grocery budget on a small list of repeat items, so tracking everything is unnecessary.

Does buying in bulk always save money?

No. Bulk buying saves money when the unit price is lower, when you have somewhere to store it, and when you finish it. If the package creates waste, the saving is negative and the money is simply spent earlier.

The bottom line

Compare groceries in two steps. First, convert the package to cost per 100 g so you can see through the sizing games. Then convert it to cost per serving so you can see through trimming, cooking and waste. The second number is the one that decides whether your grocery budget holds, and it usually takes one shopping trip to learn the handful of figures that matter.