Shopping & Pricing
Same price, smaller pack: what is the unit-price increase?
Tools to use
The shelf price is unchanged but the pack is smaller. Quantity reduction and unit-price increase have different denominators. Enter old and new packs as two candidates in Unit Price Comparison, normalize to 100 g, then optionally add a required amount to see when another pack becomes necessary.
Compare both packs per 100 g
Before: 500 JPY / 500 g = 100 JPY / 100 g
After: 500 JPY / 400 g = 125 JPY / 100 gChoose mass and a 100 g basis. Enter JPY 500, 500 g and one item for the old pack; JPY 500, 400 g and one item for the new pack. Assume tax-inclusive prices and zero shipping. Leave required quantity blank initially: unit prices are 100 and 125 yen, despite identical shelf prices.
Items per set means items included in the displayed set price, not packs you intend to buy. A two-pack for 500 yen has two items; one pack for 500 yen has one. Do not double both per-item content and item count accidentally.
Why a 20% reduction means a 25% unit-price rise
Quantity: (400 - 500) / 500 * 100 = -20%
Unit price: (125 - 100) / 100 * 100 = 25%Percentage Change gives -20% for quantity 500 to 400 and +25% for unit price 100 to 125. These measure different changes. Label the result as quantity decrease or unit-price increase rather than equating them.
The reverse is not symmetric: 125 back to 100 is a 20% decrease. Cutting 125 by 25% gives 93.75. Compare actual before-and-after unit prices instead of adding or subtracting percentage labels.
A 1 kg requirement changes purchase totals differently
Before: 2 packs, 1000 g, 1000 JPY, surplus 0 g
After: 3 packs, 1200 g, 1500 JPY, surplus 200 gEnter a required 1,000 g: the old pack needs two and the new pack three, not 2.5. Payment rises from 1,000 to 1,500 yen, but purchased quantity also rises to 1,200 g. Separate the 25% unit-price rise from the 50% increase in this particular purchase total.
The calculator does not decide whether the extra 200 g will be used or expire, nor subtract its imagined value from payment. Try requirements of 800, 1,000 and 1,200 g to see pack-count boundaries and relate the comparison to your actual use.
Add one order's shipping separately
Unit price excludes shipping. Required-quantity payment adds one order charge: three new packs plus 200 yen shipping total 1,700, not 2,100. Free-shipping thresholds and multiple orders are not inferred; enter the charge for the actual order.
Equal quantities do not guarantee equal products
Concentrated and ordinary detergent may provide different uses per 100 mL. Grams and milliliters are not interchangeable without density. Align quantity type and purpose, then assess ingredients, dose, quality and storage separately. Lowest unit price is not an overall recommendation.
Record quantity, items per set, requirement and shipping with price. Do not mix sale and regular prices or tax bases. If the previous pack is unverified, compare known products rather than inventing history. Explain the changed conditions, not just one percentage.