Why we built Sumday Carbon

Good decisions consider three things: financial consequences, social impact, and environmental cost. Most organisations will tell you they agree with that in principle. Most organisations also make the majority of their decisions based almost entirely on the first one.

That's not cynicism. It's just where the infrastructure is.

Finance has centuries of rigour behind it: standards, systems, ledgers, audit trails, people professionally trained to interrogate a number before it goes anywhere important. Social and environmental factors, by contrast, tend to arrive as narrative, as estimates, as someone's best effort in a document that lives outside the system of record.

If you want decisions to genuinely account for sustainability, not as a checkbox, not as a risk to be managed from a distance, but as a real input into how an organisation allocates money and effort, then the data has to be trustworthy enough to sit alongside the financials. That means the people making financial decisions need to understand sustainability. It means they need to trust the numbers. And it means the process of producing those numbers needs to feel like accounting, not like advocacy.

That's a wider ambition than any single product can deliver. But embedding sustainability accounting into decisions that are otherwise driven by finance alone is a strong starting point. It's where you can do the most to change how calls actually get made.

Which is why carbon accounting arrived wearing sustainability's clothes, and that was always going to be a problem.

It landed in organisations as someone else's job. Usually a sustainability manager, already overextended, doing their best with a spreadsheet, a consultant's template, and a looming board deadline. The data they needed, spend, energy, travel, fleet, lived somewhere else entirely. In finance systems. In AP ledgers. In the hands of the people whose entire professional life is organised around making numbers add up and standing behind them.

When we started building Sumday Carbon, we kept asking the same question: why isn't this a finance job?

We didn't want to build ESG software. We wanted to build sustainability accounting software, and make those two things feel exactly as different as they are.

Finance teams already have everything you need

The data that drives a GHG inventory is the same data that drives a set of accounts. Transactions. Invoices. Utility bills. Fuel receipts. Purchase categories. Finance teams don't just have access to this. They know how to work with it rigorously. They understand what audit-ready means. They know the difference between an estimate and an actual. They care deeply about the integrity of a number that will go into a report an executive signs off on.

Carbon reporting is heading exactly where financial reporting already lives: into the annual report, under the same scrutiny, carrying the same legal weight. AASB S2 is not an environmental regulation. It is a financial disclosure standard. The CFO is going to own this.

So we built software that feels less like a sustainability platform and more like a carbon ledger, familiar to finance teams, structured around the same month-end rhythms they already run, integrated with Xero so the data they need is already there. A trial balance for emissions. A general ledger for carbon. Reconciliation, not just reporting.

Sustainability leaders didn't come to their work to spend three months of the year chasing data from AP, reconciling spreadsheets, and explaining methodology to an auditor. They came to drive change: to understand what's actually driving emissions, build the supplier relationships that shift the numbers, influence the capital decisions that lock in outcomes for decades, and make the case internally for doing things differently. That's the work that matters. That's the work that requires expertise, judgment, and credibility that no software can replicate.

When carbon accounting lives in finance, in the systems, the processes, the month-end rhythms that already exist, sustainability leaders stop being the bottleneck and start being the beneficiaries. The data is there. The numbers are defensible. The audit trail is someone else's job. And the sustainability leader gets to do the work they actually came to do: use that data to drive the decisions that make it come down.

On affordability

We built Sumday Carbon to be genuinely affordable, for large organisations with complex reporting obligations, and for the small businesses that make up most of the economy and all of your supply chain. Complexity shouldn't be a barrier to starting.

Scope 3 isn't a database problem. It's a relationship problem.

This is the part we feel most strongly about, and the part that takes the most work.

Most Scope 3 solutions hand you a database of spend-based averages and call it done. You plug in your procurement categories, multiply by an emissions factor, and get a number. It's fast. It's easy. And it will not get your emissions down.

An average is a placeholder. It tells you approximately what someone like your supplier probably emits, on average, based on aggregate data from their industry. It doesn't tell you what your supplier actually emits, what they're doing about it, or whether switching to someone else would make any material difference. You can't reduce an average. You can only replace it with something real.

Real Scope 3 data comes from suppliers. Which means suppliers need to be able to measure and report their own emissions, and most of them, right now, can't. Not because they don't want to. Because the tools are too expensive, too complex, or simply never built for a business their size.

4.3 million small businesses use Xero in Australia. Through our partnership, Sumday is free for every one of them for 12 months, no strings attached.

We run supply chain programs too. When your large customers are starting to ask you emissions questions you can't answer, we help you build the capability to answer them. When you need to engage your own value chain, we give you the tools to segment suppliers, send tailored requests, track responses, and replace averages with primary data at scale.

This is not the easy route. Building a product that works for a listed entity and a two-person trades business at the same time is genuinely hard. But a database of averages isn't going to get anyone to net zero. Actual supplier data will. So that's what we're building toward.

Carbon accounting has to become normal

Not heroic. Not niche. Not the work of specialists who speak a language no one else understands. Normal, the way financial accounting is normal, embedded in the month-end close, owned by people who understand numbers and what it means for a number to be right.

That's what Sumday Carbon is for. The rigour of financial accounting, applied to emissions. Tools that finance teams can actually use. Supply chain capability that meets the reality of how businesses are structured. Education through Sumday Academy for anyone who needs to get up to speed: sustainability leads, CFOs, procurement teams, external advisors.

We're building the infrastructure for emissions data to become as reliable as financial data. That matters more than it sounds. Decisions made on bad carbon numbers are expensive at best. At worst, they're decisions that look credible on paper and change nothing at all.