Practical case 1 · One-time purchase
The product 79 Is it cheaper than the 86 € ?
Suppose the two sellers offer exactly the same new reference, with the same accessories. 79 €, then 8,90 € delivery. Offer B advert 86 €, including delivery. The known cost is therefore 87,90 € for A and 86 € for B : B costs 1,90 € less despite its higher displayed price.
Formula: price of the product + delivery + mandatory fees + necessary options. Do not add an optional option to a single offer: first compare an equivalent basket, then evaluate separately the interest of this option.
If A offers delivery from 100 €, add a product to 21 € brings the basket back to 100 €. You spend then 12,10 € more than the 87,90 This may be relevant if you already needed the second product; the free shipping threshold does not create savings on its own.
The comparison grid does not measure the risk of an unknown seller, the actual delay or the ease of a return. Take this information next to the amount. An incomplete total, for example without known delivery costs, should not be treated as the best price.
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Practical case 2 · A recurring payment
Six months to 9,99 €: what budget over a year?
Offer A costs 9,99 € per month for six months, then 19,99 € per month, plus 30 € in initial fees. Over twelve months: (6 × 9,99) + (6 × 19,99) + 30 = 209,88 €, i.e. an average of 17,49 € per month over this period.
Supply B costs 16,99 € each month without departure costs: 12 × 16,99 = 203,88 €. B comes to 6 Over six months, A comes back to 89,94 € and B to 101,94 €: the order changes according to the duration chosen, provided that it can actually stop A at this time without any additional cost.
Formula: promotional months × promotional rate + remaining months × normal rate + departure fee. Add separately the fees and equipment invoiced if they apply. Check the duration of engagement, the terms of termination and the content of the service: two subscriptions at the same price may offer different services.
The monthly average is used to compare a budget, but does not describe the monthly levies. Plan for the initial payment and the change of rate in your cash flow. The calculation uses the prices you provide; it does not provide for a future tariff revision.
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Practical case 3 · Different quantities
A package of 750 g or a batch of two packages of 400 g?
Package A contains 750 g for 5,40 € : 5,40 Φ 0,75 = 7,20 €/kg. Lot B contains 2 × 400 g, either 800 g in total, for 5,92 € : 5,92 Φ 0,80 = 7,40 €/kg. A costs less per kilo; the fact that B is sold in batch does not automatically make it more economical.
Formula: ÷ quantity expressed in the same unit. Convert grams to kilograms and millilitres to litres before division. For a lot, multiply first the quantity of a package by the number of packages. Do not directly compare a price per kilo with a price per litre.
The unit price does not assess the quality, the composition, or what you will actually use. If you use only 500 g of package A and discard the rest, the cost per kilo consumed becomes 5,40 Φ 0,50 = 10,80 €. A smaller amount more expensive per kilo can therefore be preferable when it avoids a loss.
For concentrated laundry, also compare the number of washes to the recommended dosage. For a food in a preservative fluid, check whether your comparison should relate to net weight or drained weight. Use the same convention for both products.
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Practical case 4 · Purchase and use
Does a less energy-intensive device compensate for its extra cost?
Fictitious example: A costs 400 € and uses 200 kWh/year; B costs 500 € and uses 150 kWh/year. 0,25 €/kWh, over five years and without any other cost, A is 650 € and B to 687,50 €. The annual saving of B is 12,50 €: eight years are needed to compensate for its 100 If the assumptions remain constant, the additional cost will be €.
The calculator also converts kWh/100 cycles and kWh/1 000 hours depending on your usage. It does not provide for life and does not measure the environmental balance of the replacement.
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Practical case 5 · Same amount of service
A cheaper machine for purchasing can cost more per use
Fictitious example: A costs 60 € and 0,35 € per drink; B costs 120 € and 0,20 € per drink. For 600 drinks, without any other cost in the example: A comes back to 270 € and B to 240 €. Equality is achieved 400 drinks; B becomes strictly cheaper from 401 if the assumptions remain constant.
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How to use and control a result
Take the amounts into the final basket or the contractual summary, with the date and reference of the offer. Use the same quantity, the same condition of the product, the same duration and the same services for each column. If a data is missing, keep it as unknown rather than assume that it is worth zero.
The tools perform arithmetical operations from your entry. They do not consult the prices of the merchants and do not give any commercial recommendation. The amounts displayed are rounded to the cent; for a real basket, the seller's total remains to be checked before validation.
Calculator and comparison entries stay in your browser during your visit. If a result seems inconsistent, check the unit, duration and quantity, then Report the problem with the numbers used, without bank details or personal data.
