Series B SaaS Metrics 2026 Rule of 40, NRR, magic number
Series B is where efficiency metrics catch up to growth metrics. Eight numbers dominate diligence; Rule of 40, magic number, and the GRR-NRR split move to the front of the deck. Targets below reflect the 2026 market.
1. ARR + Growth Rate
Standard Series B range. Growth rate tightens vs Series A: doubling instead of tripling. Sub-80% growth at $5-15M ARR makes the round harder to close at favourable terms.
2. NRR
Series B is where NRR really matters. Sub-105% suggests the expansion motion is not working. 115%+ NRR is what gets you a competitive Series B; 130%+ is elite.
3. Rule of 40
Series B is where Rule of 40 becomes a real diligence metric. The score (growth rate + EBITDA margin) should clear 40. Scores above 50 trade at meaningful valuation premiums.
4. Magic Number
Sales efficiency starts to matter. Magic number above 0.75 (quarterly revenue growth annualised, divided by prior quarter S&M) signals you can scale by spending more. Below 0.5 means S&M is not productive.
5. Burn Multiple
Sub-1.5x preferred at Series B. The market expects you to have learned how to spend efficiently by now. Above 2x at Series B is a major red flag.
6. LTV:CAC
Pre-Series B you got a pass on demonstrated ratio. By Series B, investors want 3x+ from real retention data (not projections). 5x+ raises questions about whether you should be spending more.
7. Gross Margin
Series B median is 75-78%. Sub-65% suggests the SaaS economics are broken; above 85% is unusual unless you have very low support cost structure.
8. GRR
Gross retention starts to get real diligence weight. NRR can mask churn through expansion; GRR cannot. Sub-85% GRR is concerning at Series B regardless of NRR.