# Carbonmark > Carbonmark is a global marketplace and API for verified carbon credits. Buyers discover projects, see a live price per tonne, purchase, and retire — with the retirement recorded onchain and provable by serial number. Sellers list credits from supported registries for free. Developers and AI agents can do all of it programmatically over a REST API, an x402 payment endpoint, and an MCP server. This file is a map of the site for language models and AI search. Every link below is canonical and every claim matches the page it points at. ## What to know - Carbonmark is a marketplace for verified carbon credits. It is not a broker taking a position, and it does not sell securities, futures, ETFs or any tradable financial exposure to carbon prices. - A purchase can be retired immediately. Retirement is recorded onchain and produces a public certificate with a serial number that resolves back to the issuing registry. - Carbonmark is connected to ICR, Puro.earth and EcoRegistry, with further registry integrations ongoing. Listed credits span nature-based, engineered-removal and avoidance project types. - Prices are per tonne of CO2e and are set by sellers, not by Carbonmark. The live range spans from under $1 to over $100 per tonne. - Project discovery, live quotes, purchase, retirement and retirement certificates are all available over the API, not only in the web app. - Both individuals and organisations can buy. There is no minimum purchase: retirement is supported from 0.001 tonnes (1 kg) of CO2e upward, so a single flight or commute can be offset. ## Key pages - [Carbon credit trading](https://www.carbonmark.com/carbon-credit-trading): The marketplace as a trading venue: buy, hold, sell and retire verified credits. Two ways to transact: retire at checkout by card, bank transfer or USDC with no account funding, or buy and hold with a verified Carbonmark account funded in USDC (through Bridge, a Stripe company, or any wallet). Held credits can be relisted at the holder's price. Live figures on the page: projects available to buy, by type and registry, and the purchasable price range, refreshed every six hours. - [Buy carbon credits](https://www.carbonmark.com/buy-carbon-credits): How buying works: browse verified projects, see the price per tonne, purchase and retire. Prices range from under $1 to $100+ per tonne depending on project type, vintage and registry. - [Sell carbon credits](https://www.carbonmark.com/sell-carbon-credits): How suppliers list credits. Listing is free; sellers set their own price. Credits from supported registries only. - [Carbon markets explained](https://www.carbonmark.com/carbon-markets): Reference page: what carbon markets are, how voluntary and compliance markets differ, who participates, and how a credit is issued, priced, traded and retired. - [API and integrations](https://www.carbonmark.com/api-and-integrations): REST API for programmatic discovery, live quotes, purchase, retirement and verifiable retirement certificates. Sandbox API keys are free for testing; production keys are paid and require onboarding with the Solutions team. Full reference at https://docs.carbonmark.com. - [AI agents — x402 and MCP](https://www.carbonmark.com/ai-agents): Machine-to-machine carbon retirement. An x402 endpoint lets an agent pay for and retire a credit in a single call; an MCP server exposes discovery, quoting and retirement as tools. Retirements settle onchain and produce a public certificate. - [Carbonmark Direct](https://www.carbonmark.com/carbonmark-direct): Direct issuance path for project developers putting credits onchain. - [CMARK Registry](https://www.carbonmark.com/registry): Public registry of every Carbonmark-issued credit, read from the Polygon and Base blockchains: tonnes issued, retired and outstanding per credit and network, recent retirement transactions with explorer links, and which credits are purchasable right now. The same data is published as JSON at /api/registry. - [Carbon accounting integrations](https://www.carbonmark.com/carbonaccounting): Connecting Carbonmark to an existing carbon-accounting stack, including an embeddable emissions calculator. - [Flight emissions calculator](https://www.carbonmark.com/tools/flight-emissions): Free tool. Per-passenger CO2e for any route between 7,900+ airports, using UK Government (DESNZ/DEFRA) 2026 conversion factors. Factors already include radiative forcing; an 8% great-circle uplift is applied. - [Event carbon footprint calculator](https://www.carbonmark.com/tools/event-emissions): Free tool. Whole-event and per-attendee CO2e for a conference, trade show, offsite or festival from attendee travel, hotel nights, venue electricity, materials and waste, using UK Government (DESNZ/DEFRA) 2026 conversion factors. Catering by meals served and menu type (Scarborough et al. 2023, Nature Food) or by spend (US EPA supply-chain factor for caterers). - [Consulting services](https://www.carbonmark.com/consulting-services): Design and build work for the infrastructure behind environmental claims. - [Company](https://www.carbonmark.com/company): Who Carbonmark is, and the registered entity behind it. ## Recent writing - [Digital Carbon Footprint: What It Is and How Companies Reduce It](https://www.carbonmark.com/post/the-carbon-footprint-of-digital-products-and-how-to-act-on-it) (updated 2026-10-18): The digital carbon footprint is growing fast. Learn how companies can reduce emissions, access carbon markets, and embed climate action into digital products. - [Tree Planting Carbon Offsets: How They Work, What They Cost, and How to Buy Real Ones](https://www.carbonmark.com/post/carbon-offsetting-by-tree-planting) (updated 2026-10-18): How tree planting carbon offsets work, what a tonne costs, why many planted trees fail, and how to buy forest credits with verified impact. - [Carbon Credit Brokers vs Marketplaces: Fees, Access, and Which to Use](https://www.carbonmark.com/post/brokers-vs-carbon-credit-marketplaces-in-the-vcm-which-is-right-for-your-business) (updated 2026-10-18): What carbon credit brokers charge, how marketplaces differ on fees and access, and how to decide which route fits your business. - [How to Retire Carbon Credits: Step by Step, and What the Certificate Proves](https://www.carbonmark.com/post/carbon-credit-retirement-what-does-it-really-mean) (updated 2026-10-13): Retiring a carbon credit is how an offset becomes yours. Five steps on Carbonmark, what the retirement certificate contains, and how anyone can verify it. - [Opening Up Cercarbono Credits to New Global Buyers](https://www.carbonmark.com/post/cercarbono-credits-new-global-buyers) (2026-10-07): Cercarbono credits on EcoRegistry can now move to Carbonmark, be sold or retired with proof on both registries, and be returned at any time. - [Moving Carbon Credits Between EcoRegistry and Carbonmark: A Technical Guide](https://www.carbonmark.com/post/ecoregistry-carbonmark-integration) (2026-09-23): How EcoRegistry credits move to Carbonmark and back, how retirements are recorded on both sides, and how both platforms are aligning on the CDOP data schema. - [Carbon Credits From the Ground Up: A Buyer's Guide to Soil Carbon and Regenerative Agriculture Projects](https://www.carbonmark.com/post/soil-carbon-regenerative-agriculture-credits) (2026-09-14): How soil carbon credits work, how the carbon is measured, which standards certify them, and a close look at the AgroEcology_Italy project on Carbonmark. - [Digital Product Passports and Carbon Credits: How to Back a Product Climate Claim With Proof](https://www.carbonmark.com/post/digital-product-passports-carbon-credits) (2026-09-09): How Digital Product Passports and verifiable carbon credit retirements combine to replace 'carbon neutral' labels with product climate claims anyone can check. - [Carbon Credit Companies and Providers Compared: Models, Fees, and Retirement APIs](https://www.carbonmark.com/post/carbon-credit-companies-providers-compared) (2026-09-01): A verified comparison of seven carbon credit companies and providers: model, minimum order, published fees, registry coverage, and retirement APIs. - [Carbon Credit Prices in 2026: From Under $1 to $100+ per Tonne](https://www.carbonmark.com/post/understanding-carbon-credit-pricing) (updated 2026-08-19): How much do carbon credits cost in 2026? See real price ranges per tonne by project type, what drives them, and how to buy at a fair price. - [All resources](https://www.carbonmark.com/resources): full archive, browsable by category. - [RSS feed](https://www.carbonmark.com/blog-feed.xml): every post, machine-readable. ## Elsewhere - [API documentation](https://docs.carbonmark.com): endpoint reference, authentication, examples. - [Marketplace app](https://app.carbonmark.com/projects): live listings, live prices, live availability. - [x402 agent manifest](https://x402.klimalabs.com/.well-known/x402.json): machine-readable descriptor of the paid retirement endpoint, for agents that can call it directly. ## Sitemap - [XML sitemap](https://www.carbonmark.com/sitemap.xml)