A trip’s carbon footprint has two parts: transportation and hotel. We calculate these items separately and then combine them to express the total footprint.

The formula for calculating the transportation footprint includes:

**The mode of transportation.** Are your attendees flying, driving, taking the train or riding a bus? The fuel efficiency of these activities varies widely, especially once we factor in the distance they’ll travel.

**Each trip’s origin and destination.** Where does each trip begin and end? This information enables us to calculate the total miles traveled. It also enables us to take secondary factors into consideration. For example, if you’re flying we’re able to use airport information to factor in the carbon effect of delays.

**The number and type of passengers traveling.** How many people in a party? Any children? As a general rule, more travelers mean more carbon footprint. However, there are some notable exceptions. For example, on flights, we assume that children under two don’t occupy their own seats or contribute to your carbon footprint in a meaningful way.

The formula for calculating your hotel footprint includes:

**Hotel location.** Our calculation incorporates the impact of the local climate and the energy required to heat or cool your hotel. We also use location to determine the fuel used to create and supply electricity to your hotel.

**Rooms and length of stay.** We multiply the footprint of a single room by the number of rooms your party will occupy and then again by the number of days you’ll stay.

One can apply a remarkable number of variables in an effort to come up with a "perfect" carbon footprint calculation. For example, take flights. Here are just some of the variables deployed in the TripZero flight footprint calculation and a brief explanation of why they matter:

**Trips**. We assume round-trip travel for every trip. For some travelers, this means we’re calculating (and offsetting) more carbon footprint than necessary.

**Distance**. Our calculation uses the nautical miles between destinations and adds in an appropriate factor to account for route inefficiencies. We do this because planes don’t typically fly directly between point A and point B; they need to spread out to avoid other flights and avoid restricted airspace (like military installations).

**Segments**. Planes use a lot of fuel during taxi, takeoff, climbing and landing. Even one stop dramatically increases your carbon footprint. For short and medium flights, we assume direct travel. For long and international flights our calculation adds an appropriate factor (based on industry averages) to account for connecting flights.

**Radiative Forcing**. With advance apology to our friends at MIT, we’ll oversimplify this important factor. Scientists agree that when you "deliver" pollution to higher altitudes (as airplanes do) you magnify its impact on climate change. Relying on a study from the Stockholm Institute of this topic, our calculation applies a factor of 2.0—effectively doubling the standard carbon footprint calculation for a flight.

**Seating Density.** Full flights, on the same plane and route, have a lower footprint per passenger than flights that are only half full. Our calculation uses industry averages to compute the effect of load factor on your flight. Relying on averages means that may understate the impact of one trip and overstate the impact of another. However across all of our customers, we’re confident that TripZero offsets the right amount of carbon footprint.

These are just a few of the variables we use to compute the carbon footprint for flights. As you might expect, we uses similarly complex models to calculate the footprint for each form of transportation and hotels.

We’ve compared the results of TripZero’s travel footprint calculations to those produced by a variety of well-regarded carbon calculators or published methodologies including:

- The US Environmental Protection Agency’s guidelines
- The TerraPass travel carbon calculator
- The Carbon Neutral Company flight calculator
- The British Petroleum carbon neutral calculator

When we conduct comparisons across bundles of trips (short medium and long), our calculation methodology typically yields total results that are somewhat higher than these models. On average, this results in our offsetting slightly more carbon footprint than customers’ trips might require.