Methodology
How the calculator turns a product's price into an estimated greenhouse-gas footprint, and then into a carbon charge you can add to the price.
What the calculator computes
For each search result it shows three numbers:
- Price — the seller's listed price, as returned by the retailer's API.
- GHG emission estimate — grams of CO₂-equivalent attributable to producing that item.
- Total = price + carbon charge — what the item would cost if a carbon price of your choosing were applied to those emissions.
The point of the third number is to collapse two dimensions into one. Most carbon tools report grams and leave you to decide what that means. Expressing the same information in money makes the comparison one-dimensional: the cheaper total is the better choice, on both counts at once.
The calculation, step by step
1. Product data
A search calls the Rakuten Ichiba API through a server-side function, which returns each item's name, price, image, link, and a genre (category) identifier. The API credentials stay on the server and are never exposed to the browser.
2. Resolving the category
Rakuten's genre tree has 4,386 nodes, and an item is tagged with a specific leaf genre. Emission factors are not defined at that resolution, so the calculator walks up the tree: it takes the item's genre path and looks for a factor at the level-2 parent, then the level-3 parent, then the genre itself.
About 1% of genres (42 of 4,386) have no factor anywhere on their path. Those fall back to the median of all mapped factors — 1.73 g CO₂e per yen — and the calculator says so explicitly in the per-item breakdown rather than hiding the substitution.
3. Applying an emission intensity
Each mapped category corresponds to a sector of Japan's 2020 input–output table, via the 3EID dataset (Embodied Energy and Emission Intensity Data for Japan). 3EID publishes, for each of roughly 390 sectors, the greenhouse gases embodied in one yen of output at producers' prices.
The estimate is then simply:
emissions (g CO₂e) = price (yen) × intensity (g CO₂e per yen)
For example, a ¥3,800 fresh flower arrangement is listed under the Rakuten genre 花・観葉植物 (Flowers & Houseplants), which maps to input–output sector 12, "Flowers and plants", with an intensity of 6.61 g CO₂e per yen:
¥3,800 × 6.61 g/¥ = 25,128 g CO₂e (25.1 kg)
4. Converting emissions into money
You choose the carbon price. The charge, and the total, follow directly:
carbon charge = emissions (g) ÷ 1,000 × carbon price (yen per kg)
total = price + carbon charge
At 20 yen per kg, the flower arrangement above carries a charge of ¥503, for a total of ¥4,303.
What is and is not included
| Included | Not included |
|---|---|
| Emissions embodied in producing the item — the supply chain that made it, including the energy and materials used by every upstream industry that contributed. | Delivery to you. Electricity or fuel the product consumes in use. Disposal and end-of-life. Emissions from land-use change and forestry, which 3EID excludes. |
This scope matters, because it is the opposite of what most people assume. Asked where a product's emissions come from, the common answer is transport. For most goods, production dominates and shipping is a small fraction. That is explained separately here.
Limitations — read these before trusting a number
- It is a spend-based estimate, not a measurement. The method is the GHG Protocol's spend-based approach: an economy-wide average for a category, scaled by what you paid. It is not a life-cycle assessment of the specific product in front of you.
- Within a category, the charge is proportional to price. Because intensity is constant across a category, a more expensive item in that category always reads as higher-emission. Totals can only reorder across categories, never within one. So the calculator can show that fresh flowers are more carbon-intensive than an artificial bouquet; it cannot tell you which of two laptops is greener.
- Category averages hide real variation. Two products in the same sector can differ substantially in reality, and this method cannot see that difference.
- Prices, not physical quantities, drive the result. A discounted item reads as lower-emission than the same item at full price. This is an inherent property of spend-based accounting.
- The genre list is a point-in-time export. Retailers add categories over time; an item in a very new genre may fall back to the median factor.
These are stated plainly because the calculator is intended to illustrate how carbon pricing changes relative costs — not to certify any product's footprint.
Choosing a carbon price
The dropdown offers a range rather than a single "right" answer, because there isn't one. Useful reference points:
| Reference | Approx. | What it is |
|---|---|---|
| Global average carbon price | USD 21/t | Average across the 87 carbon-pricing instruments in force, covering ~29% of global emissions (World Bank, State and Trends of Carbon Pricing 2026) |
| EU ETS | €60–95/t | Traded range over 2025–26 |
| Uruguay carbon tax | USD 160/t | Among the highest national rates |
| US EPA social cost of carbon | USD 190/t | 2023 estimate, 2020 dollars at a 2% near-term discount rate — an estimate of damage caused, not a market price, and highly sensitive to the discount rate |
Assuming 100 yen = 1 USD, USD 100 per tonne equals 10 yen per kg.
Data sources
Emission factors. Keisuke Nansai (2025), Embodied energy and emission intensity data for Japan using input–output tables (3EID), National Institute for Environmental Studies, Japan.
The National Institute for Environmental Studies assumes no responsibility for any disadvantage arising from use of the 3EID, and does not guarantee the validity of analyses derived from it or their interpretation. The mapping from retail categories to input–output sectors, and every figure shown by this calculator, are the author's own work and not endorsed by NIES.
Product data. Rakuten Ichiba API. Product names are returned in Japanese; category names are translated for display.