SaaS Metrics Founders Should Track: ARR, Burn Multiple and Runway

The SaaS metrics that matter most to founders are not the ones on a vanity dashboard. Revenue growth, capital efficiency and retention now decide whether a British software startup raises its next round or quietly runs out of road. This guide sets out the SaaS metrics founders should track from pre-seed through to Series A, what each number actually measures, and the 2026 benchmarks investors expect to see before they commit.

You do not need forty metrics. You need a handful you can explain from memory, defend in a board meeting, and improve month on month. The rest is noise.

Why capital efficiency now leads the conversation

The market has changed. Median annual revenue growth across SaaS has fallen to roughly 26 percent, down from about 47 percent two years ago, so pure growth no longer wins the room on its own. Investors in 2026 lead with efficiency: how much cash you burn to add each pound of recurring revenue, how long your money lasts, and whether existing customers stay and spend more. Track growth, but pair it with the efficiency numbers below or your pitch will feel dated.

Annual and monthly recurring revenue (ARR and MRR)

Recurring revenue is the foundation everything else sits on. Monthly recurring revenue (MRR) is the predictable subscription income you bill each month; annual recurring revenue (ARR) is the same figure normalised across a year. Keep these separate from total revenue, which includes one-off fees, and from bookings, which count contracts you have signed but not yet delivered.

The useful view is the breakdown: new MRR from fresh customers, expansion MRR from upgrades, contraction MRR from downgrades, and churned MRR from cancellations. Net new MRR is new plus expansion minus contraction and churn. When you can see those four moving parts, you can see exactly where growth is coming from and where it leaks away.

Burn multiple: the capital efficiency test

The burn multiple, popularised by investor David Sacks, is the single clearest measure of how efficiently you turn cash into growth. The formula is straightforward: net cash burned divided by net new ARR over the same period. If you burned 1 million pounds to add 1 million pounds of ARR, your burn multiple is 1.0.

Lower is better. A burn multiple under 1.0 is excellent, 1.0 to 2.0 is healthy for an early-stage company still finding its feet, and anything above 2.5 will worry a seasoned investor because you are spending a lot to buy a little growth. Most Series B investors in 2026 use burn multiple alongside the Rule of 40 as a first filter, so it is worth calculating yours every quarter before someone else does it for you.

Runway and burn rate

Burn rate is the net cash leaving the business each month. Runway is how many months you have left at that rate: cash in the bank divided by monthly net burn. Distinguish gross burn (total monthly spend) from net burn (spend minus revenue), because the second is the number that actually governs how long you survive.

Most guidance points to keeping 18 to 24 months of runway as a prudent buffer, and the sensible time to start raising is when you still have 18 months or more remaining. Raising with three months left is negotiating from weakness, and investors can smell it.

Net revenue retention (NRR)

Net revenue retention measures how much recurring revenue you keep and grow from your existing customer base over a year, including expansion and after accounting for downgrades and churn. Above 100 percent means your current customers alone would grow your revenue even if you never signed another logo. That is the mark of a product people rely on.

Over 100 percent is now treated as a must-have rather than a nice-to-have; 120 percent and above is elite and tends to drive materially higher valuations. For early-stage B2B startups the number is volatile because the customer base is small, so watch the trend rather than obsessing over a single month.

The Rule of 40

The Rule of 40 combines growth and profitability into one score: your year-on-year revenue growth rate plus your profit margin should add up to at least 40. A company growing 60 percent while burning at a 20 percent margin still clears the bar; so does one growing 20 percent at 20 percent profit. Be clear about which margin you use, usually free cash flow or EBITDA, and apply it consistently.

Only a minority of companies actually meet the threshold, which is exactly why investors like it as a filter. Businesses scoring above 60 routinely attract valuations two to three times higher than weaker peers.

CAC, LTV and CAC payback

Customer acquisition cost (CAC) is the fully loaded sales and marketing spend to win one customer. Lifetime value (LTV) is the gross profit you expect from that customer over the relationship. A commonly cited healthy ratio is an LTV at least three times CAC, though the more practical number for a startup is CAC payback: how many months of subscription revenue it takes to recover what you spent to acquire the customer. Under 12 months is strong for most B2B models; much beyond that and growth becomes expensive to sustain.

Churn: the metric that quietly kills growth

Churn is the rate at which customers or revenue leave. Separate logo churn (the count of customers lost) from revenue churn (the pounds lost), because losing one large account can dwarf a dozen small ones. A high growth rate can mask serious churn for a while, but eventually you are pouring water into a leaking bucket. Fixing retention is almost always cheaper than acquiring your way out of a churn problem.

How the metrics fit together

Read as a set, these numbers tell one story. ARR and its growth rate show momentum. Burn multiple and runway show whether that momentum is affordable. NRR and churn show whether it lasts. The Rule of 40 ties growth and discipline into a single figure a board can grasp in seconds. Get these right and a fundraise becomes a conversation about ambition rather than survival. For wider context on getting finance-ready and the funding options open to UK companies, the British Business Bank is a solid starting point, and investors such as Bessemer Venture Partners publish the benchmark research many term sheets are built on. For more founder guides, see the Idea London homepage.

Frequently asked questions

What SaaS metrics do investors look at first?

Growth rate, burn multiple and net revenue retention, usually in that order. Growth shows the opportunity, burn multiple shows how efficiently you chase it, and retention shows whether the revenue is durable. The Rule of 40 is often used as a quick combined filter on top.

What is a good burn multiple for an early-stage startup?

Under 1.0 is excellent and rare at the earliest stages. Between 1.0 and 2.0 is considered healthy while you are still building repeatable sales. Above 2.5 signals that you are spending heavily for limited growth and will invite tough questions in diligence.

How much runway should a UK startup keep?

Aim for 18 to 24 months of net cash runway, and begin raising while you still have at least 18 months left. That gives you room to negotiate, hit milestones, and walk away from a bad offer rather than accepting one out of necessity.

What counts as a healthy net revenue retention rate?

Anything above 100 percent means your existing customers are a growth engine in their own right. Around 110 percent is good for early B2B SaaS, and 120 percent or higher is elite and strongly associated with premium valuations.

Do pre-seed startups need to track all of these?

Not with the same precision. At pre-seed, focus on MRR growth, gross burn and runway, plus early signals on churn. Burn multiple, NRR and the Rule of 40 become far more meaningful once you have enough customers and history for the numbers to be stable.