# 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.

Phiên bản tương tác: https://www.calcopenly.com/vi/everyday/unit-price-comparison-calculator
Chủ đề: Máy tính hằng ngày

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.

## Dữ liệu đầu vào

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

## Kết quả

- Lowest unit price — kết quả chính
- Cheapest
- Đơn vị
- Saving against the dearest
- Item 1 unit price
- Item 2 unit price
- Item 3 unit price
- Item 4 unit price

## Công thức

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

## Ví dụ có lời giải

### 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%**
- Nguồn đối chiếu: 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**
- Nguồn đối chiếu: 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%**
- Nguồn đối chiếu: 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%**
- Nguồn đối chiếu: 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%**
- Nguồn đối chiếu: Hand calculation: both cost 2 per litre

## Câu hỏi

### 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” chính xác đến mức nào?

Độ chính xác phụ thuộc vào dữ liệu nhập và giả định của phương pháp. Phép tính thập phân dùng 50 chữ số có nghĩa, nhưng ước lượng, phương pháp số và dữ liệu nguồn có thể kém chính xác hơn; làm tròn khi hiển thị không loại bỏ các giới hạn đó. Ví dụ có lời giải đã đối chiếu với nguồn độc lập: 5. Ví dụ, “500 g for 3.49 against 1 kg for 5.99” được kiểm tra bằng Python decimal: 3.49/0.5 = 6.98 per kg; (6.98 − 5.99)/6.98 = 14.18%.

### Phương pháp này lấy từ đâu?

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

## Nguồn

- [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)