“Take only pictures, leave only footprints” is a common travel mantra. But how big is your carbon footprint? Several apps and websites can help you calculate that – and find ways to reduce the carbon emissions associated with your trip.

Some include an option to buy offsets – payments that go to carbon-reducing projects such as wind farms or reforestation programmes. But emissions can’t be reduced to zero simply by charging offsets to a credit card. The World Resources Institute (WRI), an environmental think-tank, cautions that while offsets make it possible to fund valuable projects, they do not lead to completely carbon-free (or guilt-free) travel. Newly planted trees, for instance, need several years to mature before they begin absorbing significant amounts of carbon dioxide. WRI advises travellers to reduce their carbon footprints as much as they can first – by booking direct flights rather than multiple legs, for instance, or checking Greenopia’s rankings of environmentally friendly airlines – and then buy offsets from certified and third-party verified projects.

But first, you need to calculate how big your carbon footprint is. Here are several recommended tools that can help.

The Conservation International’s Carbon Calculator gives a quick estimate for the carbon emissions of a single trip – based on how many miles you’re flying, how many people you’re travelling with and how many nights you’ll spend in a hotel. There’s also an option to promptly make a monetary donation to offset the emissions; for example, a 500- to 1,000-mile flight with a travel companion plus five nights in a hotel sparks a $19 donation. The US-based environmental organisation also offers smart travel tips, such as avoiding layovers as most carbon emissions occur on takeoff and landing.

Wondering which flight options emit the least carbon before you buy? Careplane, a web browser plug-in, calculates emissions as you search supported travel sites such as Kayak and Orbitz, allowing for quick carbon comparison shopping. Long haul international flights in particular can vary greatly in terms of their emissions, and the plug-in takes into account variables such as the number of empty seats on a flight (which affects each passenger’s share of the fuel burned) and the amount of cargo hauled.

Also for pre-planning, the free Flight Calculator Windows app estimates carbon emissions for flights around the world and gives an option to immediately purchase offsets that fund a range of verified projects from Carbon Footprint Ltd. For iPhone users, the Green Travel Choice app ($1.99) gives transport options for any trip anywhere in the world, with the corresponding carbon emissions for travelling by plane, bus, train, car, motorbike and more.

Travellers who are into airplane specs should look no further than the International Civil Aviation Organization. Its carbon emissions calculator, like many other sites, tracks carbon for flights – but takes into consideration industry data on aircraft type, specific routes, fuel used and passenger load. Its Green Meetings Calculator is a separate free iPhone app designed for corporate meeting planners. The app lists possible meeting locations and the carbon emissions required to get there, based on the cities from which participants are travelling. Planners of international conferences can pick a location with low carbon emissions, taking all participants’ flights into consideration.

For a carbon calculator that fits into your everyday life, the Carbon Travel Tracker iPhone app ($1.99) is designed to work in the background as you go about your travel. Turn on the app, and then check in to see your CO2 emissions by day, week, month or even year, measured both in tons of carbon dioxide and number of trees. There’s an option to name and save individual trips, and the app automatically detects whether you’re flying, driving or even cycling (which racks up zero emissions) from point A to point B. For Android users, the free Ecorio app is geared toward road-trippers and commuters. It tallies carbon emissions for ground transportation only – car or bus – and runs in the background as you head out on the highway. The app uses Google Transit to find alternative mass-transit options for your trips, and it gives you the option to instantly purchase offsets from your phone.

Calculate carbon offsets for your car, plane or train trip with the free United Nations Environment Programme Carbon Calculator iPhone app. Enter the mileage, and the app tells you what size of mangrove forest, equatorial rain forest or seagrass meadow, among other habitats, store an equivalent amount of carbon dioxide. The 220-mile train trip from New York to Boston, for instance, emits the same amount of CO2 as is stored in 2.5 cubic feet of mangrove forests. That’s about the size of a mini refrigerator.

The upstart company behind Offset4Poor, a website and free iPhone app (Blackberry and Android in development), wants your offset dollars to help the poor. Calculate carbon emissions for a road trip or flight and buy offsets to fund worldwide projects that employ those in poverty, such as tree-planting programmes that hire low-income workers, or projects that help both the poor and the environment by distributing fuel-efficient stoves, as opposed to wood-burning ones used in most homes in developing countries.