Ribbon rolls: calculate how many complete lengths your child gets
By Erik Nilsen · Kroni3 min read
A roll can be cheap per metre but provide fewer usable lengths than you expect. If each piece must be continuous, small offcuts cannot automatically be added together. Calculate complete pieces before comparing purchase prices.
Draw the cuts, not only the metres
In a hypothetical example, your child wants to make five personal cards and needs one continuous 80-centimetre ribbon for each. Roll A contains 2 metres and costs 12 units. Roll B contains 5 metres and costs 35 units. The ribbon is otherwise identical. These arbitrary prices and quantities are invented for the money exercise.
A costs 12 / 2 = 6 units per metre. B costs 35 / 5 = 7 units per metre. A initially looks cheaper. However, one A roll provides only two complete 80-centimetre pieces: 2 × 80 = 160, with 40 centimetres left.
Two A rolls provide four complete pieces, rather than five. The two 40-centimetre offcuts cannot form one continuous ribbon under this plan. You would need three A rolls costing 3 × 12 = 36 units. One B roll provides six complete pieces because 6 × 80 = 480 centimetres, leaving 20. B therefore meets the need for 35 units, one less than A.
Check the assumption about continuous pieces
If joining pieces genuinely works for the cards, consider another plan. Two offcuts might then be usable together. Do not change the assumption simply to make the cheapest roll win; your child needs to be able to use what they are paying for.
A realistic snag is your child calculating five lengths as 4 metres and therefore saying two A rolls are enough. Draw the cuts on two separate lines, one per roll. This shows why total length alone does not describe what can be cut out.
Include any extra length needed for knots before calculating. If the 80 centimetres already includes a knot, do not add it again. Measure one trial piece with material already at home if that is possible without another purchase.
Ask your family: “How many complete pieces does each roll provide, and which purchase gives us enough at the lowest total price?”
Carry the calculation into your family agreement
Kroni can keep track of your child's virtual balance. The app does not transfer money, so any actual purchase is paid for outside it. Agree on what you will record, then check the amount with your child afterwards. Do not deduct a possible cost as though the purchase had already happened. Read about allowance in Kroni.
