One grocery pack costs less to buy. Another costs less for the amount inside. Both statements can be true—and neither decides what you should put in your basket. A useful comparison separates price per unit, cash needed today and the amount you will actually use.

Create My Piggy in your browser to plan a grocery target and compare it with spending you deliberately record. No account or app download is required. This is a manual web budget, not a bank feed or a complete picture of your purchases.

For the separate same-package price example below, try your numbers in the calculator. It opens the coffee preset; you will change its unit and inputs. It does not automatically compare different pack sizes or convert grams.

Two prices answer two different questions

The package price tells you what that pack costs. A unit price expresses the cost for a common amount, such as 100 grams, so different sizes are easier to compare. NIST's 2025 Unit Pricing Guide describes using consistent units across products and pack sizes. The guide is dated June 2025; NIST's publication listing is dated July 9, 2025. Both were checked October 20, 2026. Read NIST's guide. This is educational context, not a claim about a particular store's legal obligations or an endorsement of Piggy.

Choose genuinely comparable products first: the same type and useful specification, with compatible net contents. Do not compare dry weight with liquid volume, or assume a concentrate is equivalent to a ready-to-use product. If the shelf labels use different units, calculate on one consistent basis before comparing. Piggy's scenario calculator does not perform that conversion for you.

A fictional shelf comparison: 500 g versus 600 g

Imagine two unbranded packs of the same dry pantry staple, with equivalent specification. Pack A: 500 g for USD $3.00. Pack B: 600 g for USD $3.30. These are invented teaching numbers, not observed store prices, a national average or a recommendation. Assume both amounts include any applicable tax and mandatory fees on the same basis, with no membership offer, coupon, delivery fee or discount.

Use package price ÷ grams × 100. Pack A is $3.00 ÷ 500 × 100 = $0.60 per 100 g. Pack B is $3.30 ÷ 600 × 100 = $0.55 per 100 g. Pack B costs 5¢ less per 100 g, while requiring 30¢ more at checkout.

For an arithmetic comparison at the same 600 g quantity, Pack A's unit rate would imply $3.60, compared with Pack B's $3.30. That is a reference calculation, not a claim that the store sells 600 g of Pack A for $3.60. You may have to buy whole packs. Do not describe the 30¢ reference difference as money actually saved.

Better unit value is not always the right purchase

Ask whether the extra amount fits your cash available for this shop, storage space and likely use. A bigger pack has no automatic advantage if part of it goes unused. A lower unit price also does not require you to buy more than you intended.

Keep the distinction gentle: choosing the smaller pack can be a reasonable cash-flow choice even when its price per 100 g is higher. The point is to see the tradeoff, not to grade your shopping. Check your actual balance and upcoming bills separately; a grocery target in a browser budget is not a bank balance.

A separate what-if: the same 600 g pack rises 30¢

Now stop comparing different sizes. Hold the product and 600 g package constant. Suppose that same pack moves from USD $3.30 to a hypothetical $3.60, with one pack per purchase and two purchases per week for 52 weeks. Keep taxes, fees and discounts consistent as above. This is a planning assumption, not a forecast or a purchase frequency inferred from a receipt.

($3.60 − $3.30) × 1 × 2 = $0.60 more each week. Over 52 weeks, the projected difference is $31.20, or $2.60 per average month. The old annual projection is $343.20; the scenario projection is $374.40. Dividing each by 12 gives $28.60 and $31.20 per average month. Those monthly projections cover only this assumed item and routine—not your whole grocery bill.

If you buy it for only 40 active weeks, the difference is $24.00 instead. The weekly calculator annualizes to 52 weeks; it does not know your holidays, pantry stock or changing household needs. A projection is not recorded spending or confirmed savings.

Reproduce the supported calculator example

Open the coffee preset, then replace its default example: choose USD, change Comparable unit to same-size package, enter 3.30 as Previous price and 3.60 as Scenario price, keep Units per purchase at 1, set Purchases per period to 2 and Repeat period to Week. Leave the optional monthly target blank unless you have a target for this same modeled cost. Confirm consistent units, package size and tax/fee/discount treatment.

The expected results are $2.60 more per average month, $0.60 more weekly and $31.20 more annually. We reproduced them in the live calculator on October 20. Unchecking the comparability confirmation blocks the result; a blank price produces an incomplete-input warning instead of silently becoming zero.

Do not enter the 500 g and 600 g shelf prices into that same-size comparison and confirm they match. They do not. The earlier per-100-g arithmetic is separate. Changing a currency or unit label does not convert numbers, and this calculator has no live product feed, receipt reading or automatic budget update.

Keep records and assumptions separate

For a real shopping record, check the product label, net contents, price conditions and final receipt yourself. The separate receipt-to-Excel tool can keep one selected receipt picture and reviewed English OCR details inside an XLSX. A receipt may not show package weight, quantity or a unit price clearly; do not fill those gaps with guesses. This guide uses no private receipts or actual merchant prices.

If you record a purchase in Create My Piggy, enter the amount actually paid, not the future scenario or the per-100-g reference cost. Optional saving is local to that browser and device, not cloud backup or bank synchronization. For a selected family check-in, Send Piggy PDF requires a budget and recorded entries first; preview what is included before sharing. Nothing sends automatically.

Two related questions

Is the bigger pack always cheaper? No. Calculate on the same unit and compare the actual offer conditions. Even a cheaper unit rate does not guarantee a lower checkout total or a useful amount for your household.

Can a receipt total tell me whether a product shrank? Not by itself. You need comparable net-content information from both versions, along with prices and specifications. A missing package size stays unknown. Do not infer a nationwide price trend from one basket.

Your takeaway: compare value, then choose what fits

First compare like-for-like unit prices. Then consider today's checkout cost and what you will use. Model a future price change only after choosing one compatible product and package size.

Create your grocery plan with Piggy, or try the editable same-package example in the calculator using the inputs above. Piggy is live on the web in Safari and Chrome without an app download; the native iPhone app is coming separately. Your budget is a tool for your choices, not a judgment.

Sources and coverage

Evergreen educational guide, not current grocery-price news. All products, package prices, tax/fee assumptions and buying frequencies are hypothetical; USD is an example currency, not a location-specific price observation. The live calculator supports the separate same-600-g-package price change only, not automatic pack-size normalization. NIST's document date and web listing date are distinguished. No fuel-to-food multiplier, savings promise or tax conclusion is made.

A little clarity, not a guarantee

Projections are not recorded spending or confirmed savings. No national average or future price is inferred. The web calculator does not upload receipts, send family requests, or update the iPhone app.