Unit prices: cheaper per kilogram is not always less to spend
A lower-priced pack may contain much less. A pack that is cheaper per kilogram may still require spending more than you need. Unit price reveals the arithmetic comparison; useful quantity matters to the purchase decision.
Divide price by total quantity
Unit price = total price ÷ total quantity. For a multipack, first multiply the amount per pack by the pack count. Use kilograms for every offer or grams for every offer; do not compare a per-litre number with a per-millilitre number.
Use the same currency too. Where quality, concentration or intended use differs, the calculation compares labelled quantity, not overall product value.
References: OpenStax — Ratios, rates and unit prices
Check what the price includes
Use the price actually payable under comparable conditions. Do not silently compare a conditional membership price with one available to everyone, or omit mandatory charges that change the comparison.
For a multipack, do not multiply an already-total price by the pack count again. Establish whether the label refers to one pack or the complete offer.
Theoretical savings and useful quantity
A larger pack may win on unit price while requiring more cash now. If you will not use the extra amount, not all of the apparent unit-price saving becomes a practical saving.
Keep enough decimal places while comparing small quantities and round the final display later. Early rounding can make two different offers appear identical.
Worked example: a twin pack or a single pack?
A fictional offer has two 750-gram packs for a total price of 27. Its 1.5 kilograms cost 18 per kilogram. A 1.2-kilogram pack costing 22.8 costs 19 per kilogram, so the twin pack is cheaper per unit.
If you need only 1.2 kilograms, however, the first offer requires 27 rather than 22.8 and leaves an extra 0.3 kilograms. Whether you will use it determines the practical benefit; division alone cannot answer that question.