Bulk Buy Calculator

The shelf tag says “30% off,” so the bulk option must be cheaper—right? That shortcut ignores the units that expire, the cupboard space they occupy, and the cash that could have stayed in savings. I built this calculator around a less exciting but more useful question: after waste, storage, and opportunity cost, what does each usable unit really cost?

The inputs are deliberately personal. The tool does not invent an average household waste rate. You provide the expected waste, daily usage, shelf life, storage cost, and annual opportunity-cost rate, then inspect the assumptions behind the verdict.

Start With Two Comparable Purchases

The form asks for a single-unit price, bulk unit price, bulk quantity, daily usage, shelf life, expected waste rate, monthly storage cost, and opportunity-cost rate. Calculation is disabled until the two prices, quantity, and daily usage are positive:

const canCalc = computed(() =>
  singlePrice.value > 0 &&
  bulkPrice.value > 0 &&
  bulkQty.value > 0 &&
  dailyUse.value > 0
);

The discount badge is only a headline comparison:

const discountPct = computed(() => {
  if (!singlePrice.value || !bulkPrice.value ||
      singlePrice.value <= 0 || bulkPrice.value < 0) {
    return null;
  }
  return ((singlePrice.value - bulkPrice.value) / singlePrice.value) * 100;
});

The real calculation begins with the quantity that is expected to be wasted. If you buy 20 units and enter a 10% waste rate, wasted is two and usableQty is 18. The waste rate slider runs from 0 to 80 percent, but the meaning still comes from your habits: food expiry, changing needs, duplicate supplies, and anything else that makes a purchased unit unusable.

Turn Hidden Costs Into the Same Currency

The core is a direct accumulation of costs:

const qty = bulkQty.value;
const wasted = qty * (wasteRate.value / 100);
const usableQty = qty - wasted;
const wastedMoney = wasted * bulkPrice.value;

const daysToConsume = usableQty / dailyUse.value;
const monthsToConsume = daysToConsume / 30;
const storageTotal =
  (storageCost.value ?? 0) * monthsToConsume;

const totalUpfront = bulkPrice.value * qty;
const oppCost =
  totalUpfront *
  (oppCostRate.value / 100) *
  (monthsToConsume / 12);

const totalBulkCost =
  totalUpfront + wastedMoney + storageTotal + oppCost;
const effectiveUnitCost = usableQty > 0
  ? totalBulkCost / usableQty
  : totalBulkCost / qty;

Storage is charged for the time needed to consume the usable quantity, using 30 days per month. Opportunity cost is simple annualized interest on the entire upfront bulk payment for that same duration. This is not compound interest and it does not model a changing balance; it is an intentionally understandable estimate.

The comparison baseline is the single-unit price multiplied by the full bulk quantity:

const totalSingleCost = singlePrice.value * qty;
const savings = totalSingleCost - totalBulkCost;
const roi = totalSingleCost > 0
  ? (savings / totalBulkCost) * 100
  : 0;

The result therefore answers “what if I bought this many units individually?” rather than “what if I bought only the units I will use?” That keeps the comparison fair to the actual quantity on the receipt.

Break-Even Waste Is a Sensitivity Question

The displayed break-even waste rate tells you how much waste would erase the apparent savings:

const oppCostNoWaste =
  totalUpfront *
  (oppCostRate.value / 100) *
  (monthsToConsume / 12);
const numerator =
  totalSingleCost - totalUpfront -
  storageTotal - oppCostNoWaste;
const denominator = qty * bulkPrice.value;
const breakEvenWaste = denominator > 0
  ? (numerator / denominator) * 100
  : -1;

There is a subtle approximation here. The comments in the source explicitly simplify the formula by ignoring how changing waste changes the consumption time, and therefore storage and opportunity cost. The number is a useful “how fragile is this deal?” indicator, not a mathematically exact solution to the full model.

Shelf life is checked separately. The warning computes qty - dailyUse * shelfLife, clamps it at zero, and reports leftover units. It does not subtract the waste rate in this warning, so treat it as a simple usage-versus-expiry signal rather than a second waste forecast.

The controls expose the assumptions instead of hiding them in a recommendation. Waste is a percentage slider, storage can be zero, and opportunity cost is an annual percentage slider; the English labels even explain that the waste estimate should come from your own habits. Results then show effective unit cost, wasted money, storage total, opportunity cost, both purchase totals, savings, ROI, and the break-even waste rate. That list is useful for a quick sensitivity check: if opportunity cost is nearly zero but storage is high, changing the interest rate should not magically make the deal good.

The Verdict Has a Clear Threshold

The labels are intentionally modest. Bulk buying is Worth It only when savings are positive and ROI is at least 5 percent. A negative saving is Not Worth It; all remaining cases are Borderline. That last category matters when a tiny discount disappears after one bad assumption.

Try changing only one input at a time. If moving waste from 5% to 15% flips the verdict, the purchase is sensitive to behavior, not a guaranteed bargain. Also remember that the tool rounds displayed numbers to one decimal place, so use the underlying assumptions rather than the visual rounding when comparing very close deals.

I turned this calculation into a small free tool: Bulk Buy Calculator. It is a quick way to make expiry, storage, and tied-up cash visible before a discount turns into clutter.

Comments

[Arhan Canli]

One thing in the cost accumulation looks double-counted: totalUpfront is already bulkPrice * qty, which pays for the wasted units too, and then wastedMoney is added on top. Take your 20-unit, 10% waste case at a 1.40 bulk price with no storage or opportunity cost: upfront is 28.00, wastedMoney is 2.80, so totalBulkCost is 30.80 and the effective unit cost comes out at 30.80 / 18 = 1.71. But 28.00 / 18 usable units = 1.56 already includes the waste, since you paid for 20 and used 18.

It also moves the break-even. With a single price of 2.00 the formula gives (40 - 28) / 28 = 42.9% waste. Per usable unit, 1.40 / (1 - w) = 2.00 breaks even at 30% waste. So the "how fragile is this deal" number reads about 13 points safer than it is, which is the wrong direction for a sensitivity indicator.

The baseline has a related choice: single-unit is priced at the full qty with zero waste, so the comparison assumes you would waste nothing buying singly. Buying only what you'll use (usableQty at the single price) is probably the fairer line, or at least worth a toggle.