# Unit price comparison calculator

> Unit price calculator: compare up to four pack sizes by price per kg, litre, 100 g, ounce or item to see which is cheapest and by how much.

操作できるページ：https://www.calcopenly.com/ja/everyday/unit-price-comparison-calculator
分野：日常生活の計算機

Unit price is the price divided by the size, after every pack is converted to the same unit: per kg or litre, per 100 g or 100 mL, per ounce or fluid ounce, or per item for goods sold by count. For the same product, the pack with the lowest unit price is the cheapest, and the saving is measured against the dearest pack.

With the defaults, 500 g for 3.49 costs 6.98 per kg and 1 kg for 5.99 costs 5.99 per kg, so the larger pack is 14.18% cheaper per kilogram. Packs can be entered in different units: grams, kilograms, ounces and pounds are converted with 1 oz = 28.349523125 g exactly.

Only packs sold by the same kind of measure can be compared, since weight cannot be turned into volume without the product's density. A lower unit price saves money only if you use the larger pack before it spoils.

## 入力

- **Items to compare** (選択肢：2, 3, 4)
- **Compare price** (選択肢：Per kg / L, Per 100 g / 100 mL, Per oz / fl oz)
- **Item 1 price**
- **Item 1 size**
- **Item 1 unit** (選択肢：g, kg, oz, lb, mL, L, fl oz (US), gal (US), items)
- **Item 2 price**
- **Item 2 size**
- **Item 2 unit** (選択肢：g, kg, oz, lb, mL, L, fl oz (US), gal (US), items)
- **Item 3 price**
- **Item 3 size**
- **Item 3 unit** (選択肢：g, kg, oz, lb, mL, L, fl oz (US), gal (US), items)
- **Item 4 price**
- **Item 4 size**
- **Item 4 unit** (選択肢：g, kg, oz, lb, mL, L, fl oz (US), gal (US), items)

## 結果

- Lowest unit price — 主な結果
- Cheapest
- 単位
- Saving against the dearest
- Item 1 unit price
- Item 2 unit price
- Item 3 unit price
- Item 4 unit price

## 計算式

$$
\begin{gathered}\text{unit price} = \frac{\text{price}}{\text{size}} \\ \text{saving} = \frac{u_{\max} - u_{\min}}{u_{\max}} \times 100\end{gathered}
$$

## 計算例

### 500 g for 3.49 against 1 kg for 5.99

- Items to compare: 2
- Compare price: Per kg / L
- Item 1 price: 3.49
- Item 1 size: 500
- Item 1 unit: g
- Item 2 price: 5.99
- Item 2 size: 1
- Item 2 unit: kg
- **Item 1 unit price: 6.980 per kg**
- **Item 2 unit price: 5.990 per kg**
- **Cheapest: Item 2**
- **Saving against the dearest: 14.18%**
- 照合元：Python decimal: 3.49/0.5 = 6.98 per kg; (6.98 − 5.99)/6.98 = 14.18%

### Per ounce, mixing grams and kilograms

- Items to compare: 2
- Compare price: Per oz / fl oz
- Item 1 price: 3.49
- Item 1 size: 500
- Item 1 unit: g
- Item 2 price: 5.99
- Item 2 size: 1
- Item 2 unit: kg
- **Item 1 unit price: 0.198 per oz**
- **Item 2 unit price: 0.170 per oz**
- 照合元：Python decimal with 1 oz = 28.349523125 g (NIST): 3.49/(500/28.349523125)

### 16 oz for 4.50 against 500 g for 3.20

- Items to compare: 2
- Compare price: Per kg / L
- Item 1 price: 4.50
- Item 1 size: 16
- Item 1 unit: oz
- Item 2 price: 3.20
- Item 2 size: 500
- Item 2 unit: g
- **Item 1 unit price: 9.921 per kg**
- **Item 2 unit price: 6.400 per kg**
- **Cheapest: Item 2**
- **Saving against the dearest: 35.49%**
- 照合元：Python decimal: 4.50/(16 × 0.028349523125 kg) = 9.92080…

### Eggs: 12 for 3.00 against 30 for 6.90

- Items to compare: 2
- Compare price: Per kg / L
- Item 1 price: 3.00
- Item 1 size: 12
- Item 1 unit: items
- Item 2 price: 6.90
- Item 2 size: 30
- Item 2 unit: items
- **Lowest unit price: 0.230 per item**
- **Item 1 unit price: 0.250 per item**
- **Item 2 unit price: 0.230 per item**
- **Saving against the dearest: 8.00%**
- 照合元：Hand calculation: 3/12 = 0.25, 6.90/30 = 0.23; 0.02/0.25 = 8%

### Same unit price is a tie

- Items to compare: 2
- Compare price: Per kg / L
- Item 1 price: 2
- Item 1 size: 1
- Item 1 unit: L
- Item 2 price: 1
- Item 2 size: 500
- Item 2 unit: mL
- **Cheapest: Item 1 and Item 2 (tie)**
- **Saving against the dearest: 0.00%**
- 照合元：Hand calculation: both cost 2 per litre

## よくある質問

### How do you calculate unit price?

Divide the price by the quantity in the unit you want to compare. A 500 g pack at 3.49 is 3.49 ÷ 0.5 = 6.98 per kg, or 3.49 ÷ 5 = 0.698 per 100 g. Convert every pack to the same unit first; setting a price per ounce against a price per 100 g gives the wrong answer.

### Is the bigger pack always cheaper per unit?

No. Promotions on small sizes and multipacks can reverse the usual bulk discount, so check the unit price each time. In the eggs example, 30 for 6.90 is 0.23 an egg against 0.25 for a dozen at 3.00, an 8% saving; a half-price offer on the dozen would make it the better buy at 0.125 an egg.

### What units do shelf labels use for unit prices?

In the US, the Uniform Unit Pricing Regulation in NIST Handbook 130, which states can adopt, requires stores that show unit prices to use one unit for every size of a product: per kilogram, 100 g, pound or ounce for goods sold by weight; per litre, 100 mL, gallon, quart, pint or fluid ounce for liquids; per item for counted goods. EU Directive 98/6/EC sets the unit price, including VAT, per kilogram, litre, metre, square metre or cubic metre.

### How many grams are in an ounce?

28.349523125 grams exactly, because the avoirdupois ounce is 1/16 of the international pound of 0.45359237 kg; NIST SP 811 lists the factor. A US fluid ounce measures volume, 29.5735295625 mL, so a price per ounce and a price per fluid ounce can only be compared if you know the product's density.

### 「Unit price comparison calculator」の精度はどのくらいですか？

精度は入力値と計算方法の前提に依存します。十進演算には有効数字50桁を使いますが、推定、数値計算手法、元データの精度はそれより低い場合があります。表示の丸め処理でこれらの制約がなくなるわけではありません。 独立した出典の解答と照合した計算例：5。 例えば、「500 g for 3.49 against 1 kg for 5.99」はPython decimal: 3.49/0.5 = 6.98 per kg; (6.98 − 5.99)/6.98 = 14.18%と照合しています。

### この計算方法の出典は何ですか？

NIST Handbook 130 — Uniform Unit Pricing Regulation; NIST SP 811 — exact conversion factors (1 oz = 28.349523125 g).

## 出典

- [NIST Handbook 130 — Uniform Unit Pricing Regulation](https://www.nist.gov/pml/owm/nist-handbook-130-current-edition)
- [NIST SP 811 — exact conversion factors (1 oz = 28.349523125 g)](https://www.nist.gov/pml/special-publication-811)