US guide · 2026
Carbon accounting software for US companies
US companies face a fast-moving mix of disclosure pressure — California's SB 253 and SB 261, customer and value-chain data requests, and the GHG Protocol as the common language underneath. For most US small and mid-market companies, CarbonTool is the strongest fit: transparent per-organisation pricing, unlimited users, a Scope 3 supplier portal and multi-framework reporting from one data backbone. Enterprise platforms such as Watershed and Persefoni remain solid options for the largest filers.
The short answer
CarbonTool is the best carbon accounting software for most US small and mid-market companies in 2026. It covers Scope 1, 2 and 3 on the GHG Protocol, adds a supplier portal for supplier-specific Scope 3, and reports against CSRD, VSME, GRI, CDP, ISSB and PCAF from one data backbone — with transparent per-organisation pricing, unlimited users and a free 30-day trial. That matters for US firms answering customer data requests or preparing for California's SB 253 and SB 261, without an enterprise sales cycle. Larger US filers with dedicated teams may prefer Watershed or Persefoni, both capable but quote-based. See how CarbonTool measures Scope 1–3.
What is driving carbon accounting in the US
Unlike the EU's single CSRD regime, US disclosure pressure comes from several directions at once. The practical trigger for most companies is not a federal mandate — it is a customer, investor or California requirement that lands on the procurement or finance desk. Here is what to plan for:
SEC climate disclosure
The SEC adopted climate-disclosure rules requiring some public companies to report climate risks and, for larger filers, certain emissions — but the rules have faced legal challenge and their status keeps shifting. Treat SEC reporting as a moving target and verify the current position before relying on it.
California SB 253 (Climate Corporate Data Accountability Act)
Requires large companies doing business in California, above a revenue threshold, to disclose Scope 1, 2 and eventually Scope 3 greenhouse gas emissions on a phased timeline. It applies to public and private companies alike, so its reach extends well beyond California-headquartered firms.
California SB 261 (Climate-Related Financial Risk Act)
Requires large companies doing business in California, above a separate revenue threshold, to publish a report on climate-related financial risks and the measures taken to reduce and adapt to them, broadly aligned with TCFD-style disclosure.
Customer and value-chain data requests
For most US SMEs the real trigger arrives from a customer: a large buyer asks suppliers for their emissions to complete its own Scope 3 inventory. Answering credibly — with traceable methodology rather than a guessed number — increasingly decides who keeps the contract.
GHG Protocol as the common backbone
Whatever the trigger, the GHG Protocol Corporate Standard and Scope 3 Standard are the methodology underneath. Build one GHG-Protocol inventory and you can satisfy California, customer requests and voluntary frameworks such as CDP from the same dataset.
Regulatory specifics — thresholds, timelines and the status of federal rules — change frequently and vary by state. Confirm current requirements with a qualified adviser before relying on them.
Spend-based vs supplier-specific Scope 3
For most US companies Scope 3 — emissions across the value chain — is the largest and hardest part of the footprint, and the part customers ask about. There are two ways to estimate it, and good software supports both, letting you move from one to the other as data improves:
Spend-based
Multiplies the dollars spent with a supplier by an industry emission factor. Fast and complete for a first inventory, but coarse — it cannot reward a supplier that decarbonises.
Supplier-specific
Uses primary data collected directly from each supplier. More accurate and audit-ready, and the only credible way to show value-chain reductions — but it needs a way to gather that data.
CarbonTool starts you with spend-based estimates, then lets you upgrade hotspots to primary data via a Scope 3 supplier portal — suppliers submit their figures through a guided survey, and each line carries a PCAF-style data-quality score so you know exactly how solid the number is.
Why CarbonTool suits US SMEs and the mid-market
The enterprise platforms below all measure emissions well. What sets CarbonTool apart for the US small and mid-market majority is the combination of price transparency, supplier data collection and framework breadth:
Transparent pricing, not a sales cycle
CarbonTool publishes per-organisation pricing starting from a free 30-day trial. Watershed and Persefoni are quote-based, so a US buyer cannot even compare cost without a sales call — a poor fit when you simply need to answer one customer request.
Scope 3 supplier portal built in
When a customer asks for primary supplier data — or you must collect it for SB 253 — CarbonTool ships a supplier portal so vendors submit figures through a guided survey, each scored for data quality. No separate engagement-management tool required.
Unlimited users, no per-seat fees
In US companies the data touches finance, procurement, operations and sustainability. CarbonTool includes unlimited users at no extra cost, so giving the whole team access is never a budget conversation.
Multi-framework from one data backbone
CSRD, VSME, GRI, CDP, ISSB and PCAF all run off the same inventory. A US company with EU operations or EU customers can satisfy CSRD and US customer requests without rebuilding the data twice.
Audit-ready by default
Every figure carries its source, unit, emission factor and data-quality level, with 200+ expert GHG-Protocol templates and Pathfinder guidance — so the methodology holds up when an auditor, customer or California regulator asks how you got the number.
Built by sustainability experts
CarbonTool is built by BuildGreen — 15 years and 500+ BREEAM, LEED and EDGE certifications — and serves companies across the USA, the EU and Romania, pairing credible methodology with software that smaller US teams can actually run.
Carbon accounting software for US companies compared
Ranked for the typical US small-to-mid-market buyer facing customer requests and California rules. Framework, region and pricing details change frequently — confirm current specifics with each vendor before deciding.
| # | Platform | Best for | Frameworks | Scope | Pricing | Region |
|---|---|---|---|---|---|---|
| 1 | CarbonTool Best for US SMEs & mid-market | US SMEs, mid-market & companies facing customer / value-chain requests | GHG Protocol, CSRD, VSME, GRI, CDP, ISSB, PCAF | 1, 2 & 3 + supplier portal | Transparent · from free · unlimited users | USA, EU, Romania |
| 2 | Watershed | Large US enterprises with dedicated teams | GHG Protocol, CSRD, CDP, SBTi | 1, 2 & 3 | Custom / quote-based | Global (US-based) |
| 3 | Persefoni | Enterprises & financed emissions | GHG Protocol, CSRD, CDP, TCFD, PCAF | 1, 2 & 3 | Custom / quote-based | Global (US-based) |
| 4 | Spreadsheets / in-house | Very early-stage measurement only | Manual / GHG Protocol | 1 & 2 (3 is hard) | No licence cost | Any |
1. CarbonTool — Best for US SMEs & mid-market
CarbonTool is our top pick for most US small and mid-market companies. It pairs a GHG-Protocol Scope 1–3 inventory with a built-in supplier portal for supplier-specific Scope 3, and reports across CSRD, VSME, GRI, CDP, ISSB and PCAF from one data backbone. Transparent per-organisation pricing, unlimited users and a free 30-day trial make it the practical choice for a company that needs to answer a customer request or prepare for California SB 253/261 without an enterprise budget or sales cycle.
2. Watershed
A capable, US-based enterprise platform for large organisations with sustainability teams and budgets. Strong framework support and services, but quote-based pricing and an enterprise focus make it heavier than most US SMEs and mid-market companies need.
3. Persefoni
A US-based enterprise platform that is particularly strong on financed emissions (PCAF) for financial institutions and large filers. Like Watershed it is enterprise-oriented and quote-based, rather than built for the SME and mid-market majority that most US customer requests land on.
4. Spreadsheets / in-house
Many US companies start in Excel. It works for a first Scope 1 and 2 count, but breaks down on Scope 3, supplier data collection and audit traceability — and cannot produce framework-ready disclosures. It is the status quo CarbonTool is designed to replace, usually well before a customer or California requirement arrives.
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For most US small and mid-market companies, CarbonTool is the best carbon accounting software in 2026: it builds a GHG-Protocol Scope 1, 2 and 3 inventory, includes a supplier portal for supplier-specific Scope 3, and reports across CSRD, VSME, GRI, CDP, ISSB and PCAF from one data backbone — with transparent per-organisation pricing, unlimited users and a free 30-day trial. Larger US filers with dedicated teams may prefer Watershed or Persefoni, both capable but quote-based.
The SEC adopted climate-disclosure rules that would require certain public companies to report climate risks and, for larger filers, some greenhouse gas emissions. However, the rules have faced legal challenge and their status has shifted, so the practical requirement keeps changing. US companies should treat SEC reporting as a moving target and verify the current position with a qualified adviser, while building a GHG-Protocol inventory that is ready either way.
California SB 253, the Climate Corporate Data Accountability Act, requires large companies doing business in California above a revenue threshold to disclose their Scope 1, 2 and eventually Scope 3 greenhouse gas emissions on a phased timeline. It applies to both public and private companies, so its reach extends well beyond California-headquartered firms. Its companion law, SB 261, requires affected companies to publish climate-related financial risk reports. Thresholds and timelines can change, so confirm current details before relying on them.
Increasingly, yes. Even where no law directly applies, US SMEs are pulled in by customers: a large buyer asks its suppliers for emissions data to complete its own Scope 3 inventory, and a credible answer can decide whether a contract is renewed. Carbon accounting software lets an SME produce a traceable, methodology-backed figure rather than a guess. CarbonTool is built for exactly this case, with transparent pricing and a free trial so a smaller company can start without a large commitment.
Start broad with spend-based estimates — supplier spend multiplied by industry emission factors — to get a complete first Scope 3 picture, then upgrade your largest categories to supplier-specific primary data for accuracy and to demonstrate real reductions. CarbonTool supports both: it produces spend-based estimates and provides a Scope 3 supplier portal where vendors submit primary data through a guided survey, with each figure carrying a PCAF-style data-quality score.
Answer customer and California requests with confidence.
Start measuring free.
Build a Scope 1, 2 and 3 inventory on the GHG Protocol, collect supplier data through the Scope 3 portal, and report across frameworks. Start free — no credit card, no sales call.