usefulweb.site Bulk Buy Brain

Money-saving & comparison

Bulk Buy Brain

Is the bulk pack really cheaper once waste, storage and tied-up cash are counted?

Shown in results only; no conversion is performed.

weeks

Leave empty for non-perishables.

per week

Optional: freezer energy, rented space…

%/year

Try an example: Toilet paper: 8 rolls vs 48 rolls · Greek yoghurt: 500 g vs 2 kg tub (spoils)

How it works

Compare a small pack with a bulk pack using how fast you actually use the product, its shelf life, storage cost and the interest your money could earn. The tool computes the effective cost per unit you actually use, your saving or loss, and the largest bulk size worth buying.

  • Small-pack unit price ps = small price ÷ small quantity (small packs bought as needed, no waste). Bulk unit price pb = bulk price ÷ bulk quantity.
  • Usable quantity = min(bulk quantity, consumption per week × shelf life); waste = bulk quantity − usable quantity.
  • Weeks held = usable quantity ÷ consumption per week. Storage cost = storage cost per week × weeks held.
  • Opportunity cost = bulk price × interest rate × (weeks held ÷ 52.18) ÷ 2 (the prepaid amount is drawn down linearly, so on average half is tied up).
  • Effective cost per used unit = (bulk price + storage + opportunity cost) ÷ usable quantity. Savings = ps × usable quantity − bulk total cost.
  • Max worthwhile size: the largest Q for which pb·Q + storage + opportunity cost ≤ ps × usable(Q), solved in closed form for the no-waste and the waste regime.

For scripts and AI agents

  • JSON API: POST https://bulkbuybrain.usefulweb.site/api/v1/calculate (same calculation as this page)
  • Interface description: OpenAPI 3.1 · AI guide: llms.txt
  • MCP server (Streamable HTTP): https://bulkbuybrain.usefulweb.site/mcp · tool name bulk_buy_brain
Example request
curl -s -X POST https://bulkbuybrain.usefulweb.site/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{"currency":"EUR","unitLabel":"rolls","smallPrice":3.49,"smallQuantity":8,"bulkPrice":17.99,"bulkQuantity":48,"consumptionPerWeek":3,"storageCostPerWeek":0,"interestRatePercent":3}'
Feedback