Your flowers are organic, your dresses are vintage, and you’ve asked your caterer to forego the plastic silverware and paper plates. You’ve planned a menu of local food and set up buses to transport your guests to minimize the number of cars they drive.

You’ve done everything you can to make your wedding day environmentally friendly, but you’re still worried about the impact of the things you can’t “green” -- your guests travel to your wedding or the hotel where they’re staying, perhaps.

Enter carbon offsets or credits. Essentially, carbon offsets are money. You’re paying money toward green energy projects to make up for polluting. Most commonly, offsets are purchased by companies, factories, or governments to comply with carbon dioxide emissions caps. When a company goes over its emissions cap, it buys offsets to cover the amount it exceeded the limit.

In the last couple of years, though, it’s become common for individuals to buy offsets for behavior such as driving, flying, home energy use, or events like weddings. Here’s how offsets work:

You choose one of the many firms that sell carbon offsets (we’ve profiled four below), and use their calculator to figure out the impact of your wedding in metric tons (2,205 lbs.) of carbon dioxide emissions. You can buy offsets to cover all or part of that impact. Your money goes toward renewable energy or emissions-reducing projects.

Each offset seller computes your impact in different ways, but they all look at guest travel by air and car and number of hotel nights. The prices range from $8 to $12 per metric ton, and some firms even let you choose which project or type of project you’d like to benefit. You can read about past and current projects at their websites.

Here are four companies that sell carbon offsets specifically for weddings and events:

CarbonFund.org
Silver Spring-based CarbonFund is the only non-profit out of the four companies we’re profiling. They support renewable energy projects that focus on clean energy development, energy efficiency projects that work toward reducing existing use, and reforestation. CarbonFund lets you choose what project you’d like your offsets to support.

Calculator: Number of guests, number of guests flying, average flight length, number of guests driving, average driving distance, and number of hotel nights. To cover the rehearsal dinner, 1,000 lbs. of CO2 is added.

Cost: $10 per metric ton

Website: http://www.carbonfund.org/site/pages/individuals/category/Weddings/

DrivingGreen.com
This international company (they have offices in Ireland, the U.S., Canada, Brazil and Mexico) has a generic event calculator that can be used for weddings. They have a specific focus: projects that reduce methane and nitrous oxide emissions in agriculture. The gases are converted to energy that can be used to run the farm.

Calculator: Number of 1- to 4-hour flights, number of 4- to 7-hour flights, number of international flights, number of guests driving less than 100 miles, number driving 100 to 500 miles, number driving 500 miles or more, and number of hotel rooms (based on a two-night average).

Cost: $8 per metric ton

Website: http://www.drivinggreen.com/event.asp

NativeEnergy.com
NativeEnergy focuses on projects that benefit Native Americans, Alaska Native Villages, family farms and rural communities. Based in Charlotte, Vt., NativeEnergy offset the film and book An Inconvenient Truth as well as Al Gore’s travel. Their wedding calculator is done in partnership with Portovert, a green wedding magazine. You can choose how your offsets are used: 100 percent wind, 100 percent methane or a 50/50 split.

Calculator: Number of guests, number of days of event, number of hotel nights, what state your wedding is in, square footage of wedding site, square footage of reception site, number of people flying and the average flight length, and the number of people driving and the average driving distance.

Cost: $12 per metric ton

Website: http://www.nativeenergy.com/pages/portovert/168.php

TerraPass.com
Based in San Francisco, TerraPass funds three projects types: clean energy production by wind power, farm power using animal waste, and landfill gas capture, which reduces the impact of our trash. If you offset your wedding with TerraPass, they send you a certificate you can display at your wedding.

Calculator: Number of short flights (less than 2 hours), number of medium flights (about 4 hours), number of long flights (more than 4 hours), number of cars driven to event, average one-way trip, and number of hotel room nights. They add a little on the top to cover energy use for your wedding.

Cost: About $12.70 per metric ton

Website: http://www.terrapass.com/wedding/weddingcalc.php