Click the Calculate Emissions link on the left (if needed) and then click the Single Trip tab at the top of the calculator. Next to Auto Travel, enter your “Round-trip miles” and “MPG,” and then click Calculate. We even provide a link to a distance calculator where you can calculate distances between cities. If you then want to offset the CO₂ from your trip, click the Purchase Now button and follow the instructions.

The lbs. of CO₂ per gallon for diesel is 22.384 lbs. per gallon. The lbs of CO₂ for gasoline is 19.564 lbs. of CO₂ per gallon. So your can multiply your miles by 22.384/19.564 = 1.144 to adjust your miles correctly for diesel.

Since diesel cars get better mileage than gas cars, CO₂ emissions per mile are less for diesel. Example: Two cars, one gas powered and one diesel powered, make the same 300-mile trip. The gas car gets 25 MPG and uses 12 gallons of gas, while the diesel car gets 45 MPG and uses 6.66 gallons of diesel. Using the conversion number in the question above, we get 234.77 lbs. of CO₂ for the gas car and 149.23 lbs. of CO₂ for the diesel car.
Please note: diesel cars in the US produce more pollutants than gas cars, so total CO₂ emissions do not account for the total impact of a diesel car.

See the question above to correct the mileage for diesel operation. Since biodiesel is 25% methanol, which comes from fossil fuels, you can reduce your total miles by 75% (total miles * .25) after you account for diesel use. This is true only if you use biodiesel made from waste (used) oil. If you are using biodiesel made from soy or canola oil, you should reduce your corrected diesel miles by about 50% to account for the fossil fuel used to fertilize, grow, transport, and process the oil into biodiesel.
Note: while biodiesel is much cleaner then regular diesel (much fewer particulates, hydrocarbons, and no heavy metals) it does produce more NO_x than regular diesel. Thus, CO₂ emissions are not the only thing to look at when purchasing a diesel car to run on biodiesel. Also, check the availability of biodiesel in your area.

You need to figure out the passenger miles per gallon (MPG) for your mode of transportation and use this for the MPG in the calculator. Buses get about 30 MPG per passenger mile, while trains get 100 to 200 MPG per passenger mile. The actual passenger MPG will also vary with the number of passengers. Remember that a good guess is better than none, so don't worry too much about the exact numbers if you want to offset your CO₂ from these modes of travel.

This is a common question, and at first glance seems impossible, but when you look at the chemistry of gasoline combustion, it actually makes sense. CO₂ is one part carbon and two parts oxygen. A gallon of gas is about 6.25 lbs. at 72 degrees and is mostly carbon with hydrogen, which is very light. So, taking into account the 1:2 ratio of carbon to oxygen for CO₂, and the atomic weights of carbon (12) and oxygen (16), we get 19.564 pounds of CO₂ for each gallon of gas. Let's look at this more closely.

For one gallon of gas weighing 6.25 lbs., carbon makes up 84% of the weight of the gas. This assumes that that gas is a simple hydrocarbon, C7H16. This comes out to 5.25 lbs. of carbon. CO₂ is one part carbon and two parts oxygen, so we get a 1:2 part ratio. If the weight of carbon and oxygen were the same, we would get 3 * 5.25 = 15.75 lbs. of CO₂ for each gallon of gas burned. The atomic weight of carbon is 12 and the atomic weight of oxygen is 16, so the actual weight of CO₂ is one part C (12) and two parts O (16). Thus, the total weight would be 5.25 lbs. of carbon + 2 * (5.25lbs.) * (16/12) = 5.25 + 2 * (7.00) = 19.25 lbs. This assumes that gasoline is just a simple hydrocarbon, C7H16, which it is not. Gasoline has many other hydrocarbons and additives, so taking this into account, one gallon of gas burned produces 19.564 lbs. of CO₂. For more than you ever wanted to know about gasoline, see: http://www.turborick.com/gsxr1127/gasoline.html

Click the Calculate Emissions link on the left (if needed) and then click the Single Trip tab at the top of the calculator. Next to Air Travel, enter your “Round-trip miles” and then click Calculate. We even provide a link to a distance calculator where you can calculate distances between cities. Or, if you prefer, you can use the national average of 1,660 miles/trip by clicking the box. If you then want to offset the CO₂ from your trip, click the Purchase Now button and follow the instructions.

The primary reason is that airplanes fly at a high altitude, and therefore deliver their vapor trails and pollution into the most vulnerable part of the atmosphere. Therefore, people who fly a lot exacerbate this problem and generate a lot of CO₂ as well. That said, the miles per gallon per passenger for air travel are not too bad when comparing CO₂ emissions. It's about the same as driving a Toyota Prius.

Often this information is on your plane ticket or reservation for your frequent flier miles. If you can't find it there, you can use any of the following sites to calculate your mileage between cities.
http://www.webflyer.com/travel/milemarker/
http://www.usatoday.com/travel/flights/miles/calculator.htm
http://www.airtimetable.com/Air_mile_calculator.htm
http://www.geobytes.com/CityDistanceTool.htm

Some other carbon calculators include CO₂ equivalent (CO₂e) to account for other damaging emissions from air travel. Since we only calculate CO₂ (and not CO₂e) in other areas of the calculator, we decided to calculate only CO₂ for air travel as well.

Aviation fuel produces 18.355 lbs. of CO₂ per gallon. The best way to figure this would be to treat the air travel like auto travel and calculate the total annual miles traveled and the plane's MPG. Then multiply the total miles by 0.938 (18.355/19.564) and enter that number, and the MPG, on the Auto Travel page.

The most accurate way is to add up your monthly electricity use over the past 12 months and divide by 12 to get a monthly kWh average. Then enter this kWh number on the Electricity page and click Calculate.

Our partner, 3Degrees Inc., also provides RECs for the PaloAltoGreen and Santa Clara Green Power energy programs. Since the price of "green energy" through these municipal utilities is the same as the price of Cool It! RECs, we are encouraging residents of these two cities to support their local green power programs.

One CFL uses 1/4 the electricity of a regular incandescent bulb. So, with a 100-watt light bulb you are saving 75 watts. If this bulb is on 8 hours a day (3.5 hours in the morning and 4.5 hours at night), the energy saved each year by using it is 75 watts x 8 hours x 365 days = 219,000 watt-hours or 219 kWh. This number equates to 143 lbs. of CO₂ per year in California, or 11.9 lbs. of CO₂ per month in California. Your calculations will vary depending on how many hours a day your CFL is on, the wattage of the bulb, and what state you live in.

Note: the amount of CO₂ generated per kWh in California is about 1/4 the national average, since California uses less fossil fuel (primarily coal) and more non-carbon based sources (hydro and nuclear) to make electricity - so choosing a different state could make a big difference.

No. You are producing renewable energy already and don't need to account for this in your CO₂ emissions calculations. There is some energy used in the making of solar panels, and therefore some CO₂ emission, but this is “paid off” in the first two or three years of operation. Thanks for being part of the solution to our energy and global warming problems!

The most accurate way is to add up your monthly heating fuel use over the past 12 months and divide by 12 to get a monthly average. (Or, you can use the national average by clicking the box.) Then enter this number in the “Therms/month” or “Gallons/month” fields on the Heating page and click Calculate.

To make this calculation, you need to decide what activities you consider part of the event. For example, CO₂ is generated by the attendees traveling to and from the event, at the event itself, and during any overnight hotel stays. So, a full calculation of emissions would involve all of these activities. If you are interested in doing a full calculation, please contact Debbie Mytels (debbiem[at]acterra[dot]org or 650-962-9876 x302) and she can help you with the process.

Here's the basic process by activity area:

Auto Travel: Calculate or estimate the total miles driven by attendees and an average mile-per-gallon (MPG). On the Auto Travel page, enter this information in the “miles/year” and “MPG” fields, and then click Calculate.

Air Travel: Calculate or estimate the total miles flown by attendees. On the Air Travel page, enter this amount in the “Air miles/year” field, and then click Calculate.

Event Electricity: Calculate or estimate the total kWh used during your event and divide this number by 12. On the Electricity page, enter that amount in the “kWh/month” field, and then click Calculate. To estimate how many kWh your event will use, ask the Facilities manager at the event location for an approximate number. If there's a dedicated electricity meter for your location, you can calculate your kWh usage more accurately by reading the meter before and after the event, dividing the difference by 12, and entering that amount in the “kWh/month” field.

Hotel Electricity: Multiply the number of hotel rooms occupied by the number of days occupied by 14.7 kWh/room/day. Divide this number by 12, enter it in the “kWh/month” field on the Electricity page, and then click Calculate.

Note: If you are calculating electricity usage for both the event and hotel stays, add the two kWh numbers together and enter a single number in the “kWh/month” field on the Electricity page.

Heating: Calculate or estimate the total amount of natural gas, propane, or heating oil used during your event and divide this number by 12. On the Heating page, enter the amount(s)s in the “Therms/month” or “Gallons/month” fields, and then click Calculate. To estimate how many therms or gallons your event will use, ask the Facilities manager at the event location for an approximate number. If there's a dedicated natural gas, propane, or heating oil meter for your location, you can calculate your usage more accurately by reading the meter before and after the event, dividing the difference by 12, and entering that amount in the appropriate therms or gallons fields.

For a list of additional resources, including web sites on energy efficiency, click here.