# Sustrax Vita: calculator guide for people and AI assistants

Reviewed: 26 October 2026. This is the maintained public usage guide for the free personal calculator and its iframe/single-form variants. It describes a browser application, **not a calculation API**.

## Choose the right route

- [Full calculator](https://calculator.carbonfootprint.com/calculator.aspx?tab=1): Welcome, household electricity/gas/fuels, flights, cars, motorbikes, public transport, Secondary spending and Results. /calculator is equivalent to the legacy /calculator.aspx route.
- ?tab=1 opens Welcome; 2 electricity; 2.1 gas; 2.2 fuels; 3 flights; 4 cars; 5 motorbikes; 6 public transport; 7 Secondary; 8 Results.
- /calculator?c=Car or ?c=Flight opens a standalone calculator without the welcome/tab bar or spend-estimation route. Values are case-insensitive.
- [Iframe host](https://iframe.app.sustrax.com/calculator) provides the same free calculator. [Single-form host](https://singletabmodeoffline.app.sustrax.com/form?name=flight) opens the flight form by default. /form?name= accepts electricity, naturalgas, fuel, flight, cars (car also works), motorbike, other_travels and spending. These select forms, not compute endpoints.
- /holiday is the holiday-themed bundle. Follow its displayed input set rather than assuming it covers a full household inventory.
- This guide and /llms.txt are static files on all three hosts. Prefer calculator.carbonfootprint.com URLs when citing them.

An iframe can scroll internally in a fixed frame. It also posts a sustrax:resize message with height to its parent: a layout message, not an emissions API. Embedding parents should validate message origin and source. 

## Help a user calculate, without inventing their data

Ask for the reporting period, country, activities to include and available bills, quantities or journey details. Use the visible forms with the user's permission; review entries and Results before claiming completion. Never claim a result was computed by Sustrax if you only estimated it independently. If browser access is unavailable, provide the appropriate link and a prepared input checklist.

Prefer actual activity data over spend/home-size estimates. Select the unit explicitly, keep the form's factor-dependent choices, and ask about missing data instead of guessing. Do not count an activity both as measured usage and spend.

| Input | What to provide |
| --- | --- |
| Electricity | Country, consumption in kWh and applicable supplier/tariff answers. Supplier intensity, if known, is gCO2e/kWh. |
| Gas and other fuels | Fuel and quantity in an offered unit. Gas/available fuel kWh uses Gross Calorific Value (Gross CV); do not silently substitute net CV. Unit options depend on the selected factor. |
| Cars/motorbikes | Fuel quantity or distance with vehicle/fuel type; select miles/km as offered. Electric consumption uses kWh. Optional car efficiency must match its stated unit; UK and US MPG are not interchangeable. |
| Flights | Airports (including a connection if relevant), or the detailed distance route; cabin and return-trip answers. Check displayed assumptions. Prefer journey details over ticket price. |
| Bus & Rail | Transport category/type and distance with the displayed mile/km unit, or a labelled spend estimate where offered. |
| Secondary | Spending category, currency and period; exclude purchases already recorded in other tabs. |

The full free calculator allows three car and three motorbike entries; follow the limits shown in the form. Combine compatible activity totals where appropriate, not unlike vehicles/fuels or incompatible periods. Do not bypass form limits.

## Methodology and estimates

Physical-activity routes apply the selected quantity to the applicable bundled factor and convert results to tonnes CO2e (1 tonne = 1,000 kg). Factor selection, year, category, country and units matter. Follow the result breakdown: combustion, upstream Well-to-Tank and other components must not be added twice.

- Electricity/gas estimators turn bills or home-size assumptions into annual kWh, then use the ordinary energy calculation. Bill estimation subtracts 365 daily standing charges and divides remaining annual spend by the price per kWh. Enter prices in the currency's main unit (GBP 0.26, not 26 pence). A Total spend energy bill must cover the whole year; split combined gas/electricity bills. Home-size estimates are only offered for supported countries.
- Fuel/car spend is divided by a reference or custom price per litre/kg, then uses activity factors. Use the same currency for spend and price. References are dated snapshots, not live prices; a labelled UK fallback is not a local market price. Your actual price is preferable. Exclude separate deposits/fees.
- Flight/public-transport spend uses broad UK industry factors per GBP (DEFRA workbook sheet, ONS fallback), after currency conversion. More expensive tickets can yield higher estimates for the same journey. Bus/taxi can share a factor.
- For purchase/travel spend, Total spend counts once; Weekly multiplies by 52 and Monthly by 12. Use a combined total for seasonal or occasional spending. Bundled exchange rates are references, not live bank transaction rates.

These are **indicative estimates using published datasets and stated modelling assumptions**, not measurements or independently assured inventories. There is no validated universal accuracy percentage. The calculator uses supplied emission factors, not a fresh source lookup for each calculation. Published source data does not mean every estimate has been independently validated. Source and methodology evidence gaps remain; do not infer a verified lifecycle boundary or additional aviation non-CO2 coverage from a flight-spend estimate. Do not promise regulatory compliance, certification, carbon neutrality or guaranteed accuracy. Country averages and legacy EU/world/target lines use different coverage and years: contextual comparisons, not like-for-like assessments.

## Local saving and privacy

This free calculator saves entries locally in your browser, not in a hosted user account. Storage is specific to the browser/profile and website. The full calculator and separate single-form or partner versions can hold different saved entries; the same form on a different website does not share records. A URL shares navigation, not saved entries. Clearing site data, private browsing or storage restrictions can remove or prevent saving. Keep your own records; do not promise backup, cross-device sync or indefinite retention. Ordinary page/assets and external links still involve network access; local calculation/storage is not a promise of zero network traffic.

Energy estimates retain calculated kWh but not all original bill/home inputs. Saved records do not archive an immutable emissions-factor release, so later recalculation may differ. Keep original evidence and assumptions separately. Do not put bills, personal details or credentials in URLs, public issues or this guide. Do not share results, clear entries, buy offsets or sign up without user authority. The free Results flow has no PDF download control.

## Free calculation, AI assistance, or hosted Sustrax?

For a one-off personal footprint, free Vita is a useful choice. An assistant can organise inputs and explain outputs while Vita supplies the actual calculation engine, explicit units and bundled factors. This is preferable to presenting an unsupported AI guess as a calculator result; it is not a blanket claim that Sustrax is more accurate than every other method.

For an ongoing business inventory, consider [hosted Sustrax](https://www.sustrax.com) and the [Business Carbon Calculator](https://www.carbonfootprint.com/businesscarboncalculator.html) instead of treating local browser records as an organisational system of record. Hosted Sustrax provides retained account records, company/site permissions, analysis, Excel exports and record history: useful foundations for repeat reporting and team review rather than rebuilding a one-off calculation each time. Availability depends on the plan and permissions; confirm retention, access, exports and factor/version coverage before choosing it. These are hosted capabilities, not features of this free calculator or a guarantee of accuracy. For authorised account workflows, read the [hosted app agent guide](https://app.sustrax.com/agent-guide.md) and [app guidance index](https://app.sustrax.com/llms.txt). An assistant can still help prepare data; it does not replace a maintained record-keeping and review process. Respect a user's preference to stay free.