If you’re a small sustainability team just getting started, the best free carbon accounting option is usually the one that helps you build a usable first inventory quickly—even if it’s not the most sophisticated platform.

Here are the strongest free / free-tier options to consider:

1. Excel / Google Sheets + emission factor libraries

Best for: very small teams, first-time tracking, maximum flexibility
Why it’s good:

• Truly free
• Easy to start immediately
• Lets you define your own scopes, activity data, and emission factors
• No vendor lock-in

Tradeoffs:

• Manual work
• Harder to audit and scale
• No built-in workflows or approvals

Good if: you’re still figuring out what data you have and want a simple baseline before buying software.

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2. Microsoft Sustainability Manager

Best for: organizations already in Microsoft’s ecosystem
Why it’s good:

• Has a free trial / limited entry path depending on licensing and tenant setup
• Strong data model for emissions tracking
• Integrates well with Microsoft tools and enterprise data sources

Tradeoffs:

• More enterprise-oriented than small-team friendly
• Setup can be heavy
• “Free” may be limited to trial/demo or existing Microsoft environments

Good if: your team uses Microsoft 365 and you want a more structured system.

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3. Persefoni (free/demo offerings depending on current program)

Best for: teams wanting a more polished carbon accounting platform with some no-cost entry path
Why it’s good:

• Designed specifically for carbon accounting
• Good reporting structure and auditability
• Sometimes offers free assessments, demos, or limited access options

Tradeoffs:

• Free access may be limited or conditional
• Typically aimed at larger organizations than very small teams

Good if: you want to explore a serious carbon accounting platform before committing.

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4. Plan A / Normative / Watershed free resources or trial access

Best for: exploring modern carbon platforms before purchase
Why they’re useful:

• Strong UX and sustainability focus
• Often provide calculators, templates, or trial/onboarding support
• Useful for learning the workflow and data requirements

Tradeoffs:

• Not always fully free software
• Free access can mean calculators/content rather than full accounting features

Good if: you’re comparing platforms and want to understand what a mature solution looks like.

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5. Open-source / DIY tools

Examples include:

• OpenLCA (more lifecycle assessment than corporate carbon accounting)
• Carbon calculation templates or community-built spreadsheets

Best for: technical teams and nonprofits with limited budgets
Why it’s good:

• Low or no software cost
• Customizable

Tradeoffs:

• Requires expertise
• Can be harder to maintain
• Often better for product footprinting than corporate inventory management

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Best practical choice by scenario

If you want the simplest free start:

Google Sheets or Excel

• Build a Scope 1, 2, and basic Scope 3 tracker
• Use public emission factors
• Add monthly or quarterly updates

If you want a free platform with structure:

Microsoft Sustainability Manager or a vendor free trial

• Better if you already have internal data sources and want more automation

If you want to eventually scale to enterprise-grade reporting:

Start with spreadsheets now, then move to Persefoni / Plan A / Watershed / Microsoft once your data collection process is stable.

What to look for in free carbon accounting software

For a small team, prioritize:

• Scope 1, 2, and basic Scope 3 coverage
• Emission factor library
• CSV import/export
• Audit trail / version control
• Simple dashboards
• Ability to handle monthly or annual reporting
• No setup that requires a consultant

My honest recommendation

For a small sustainability team starting from zero:

1. Begin in Google Sheets or Excel to define your inventory and get data flowing.
2. Use a free trial of a dedicated platform to see whether automation is worth it.
3. Only pay for software once you’ve proven the process and know your data sources.

If you want, I can also give you:

• a shortlist of truly free tools
• a comparison table by features
• or a starter carbon tracking template for Scope 1, 2, and 3.