Revenue Forecast Formula for SaaS: Build a Defensible Model

The revenue forecast formula for SaaS is more specific than a generic business model: it starts with current MRR, adds expected new revenue, subtracts churn, and adds expansion. That four-component structure, starting MRR + new MRR + expansion MRR − churned MRR = ending MRR, is the foundation of every defensible SaaS revenue forecast. The hard part is not the formula. It is getting accurate inputs for each component and building a range of scenarios that reflects real uncertainty. This guide covers the formula, how to get each input, worked examples, and how to present a forecast you can actually defend.

Revenue Forecast Formula (SaaS) = Starting MRR + New MRR + Expansion MRR − Churned MRR − Contraction MRR = Ending MRR. Project this forward by month to build a 12-month revenue forecast.

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Revenue Forecast Formula for SaaS

To build the full financial model, see saas financial model the minimal sheet that predicts runway.

The Basic Formula

Ending MRR = Starting MRR + New MRR + Expansion MRR − Churned MRR − Contraction MRR

Variable definitions:

• Starting MRR = recognized MRR at the beginning of the period (your current baseline)
• New MRR = MRR from customers who did not exist in the prior period (new sales)
• Expansion MRR = MRR added from existing customers (upgrades, seat additions, usage growth)
• Churned MRR = MRR lost from customers who canceled
• Contraction MRR = MRR lost from existing customers who downgraded (still customers, just paying less)

For a 12-month forecast, you apply this formula 12 times in sequence, each month’s ending MRR becomes the next month’s starting MRR.

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Step 1: Establish Your Starting MRR

Your starting point is your current recognized MRR. Pull this from your Stripe dashboard, billing system, or analytics tool. Make sure you are using recognized MRR (monthly recurring revenue for the current month) not total cash collected (which includes annual prepayments).

Common errors:

• Using total Stripe charges instead of normalized MRR (annual plans paid upfront distort the number)
• Including one-time charges (setup fees, professional services) in MRR
• Counting trials or free plans as MRR

If you have €32,500 MRR today, that is your starting MRR for the forecast.

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Step 2: Forecast New MRR

New MRR projection requires honest assumptions about your sales velocity.

Approach 1. Historical run rate:

Look at the last 3–6 months of new MRR added. Take the average or median. Use that as your monthly new MRR assumption.

If you added €2,800, €3,100, and €2,500 in new MRR over the last three months:

Average new MRR = (€2,800 + €3,100 + €2,500) ÷ 3 = €2,800

Use €2,800/month as your base case new MRR assumption.

Approach 2. Pipeline-based:

If you have a sales pipeline, apply close rate assumptions:

New MRR = Pipeline MRR × Expected close rate × Timing factor

For example: €15,000 in pipeline, 30% close rate, expect 70% to close this month:

New MRR = €15,000 × 0.30 × 0.70 = €3,150

For more on growth modeling inputs, see SaaS forecast model.

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Step 3: Forecast Churn

Churn is the most important variable in any SaaS revenue forecast. A small change in churn rate has an outsized effect on 12-month MRR because it compounds.

Formula:

Churned MRR = Starting MRR × Monthly Churn Rate

Use your historical average monthly churn rate, calculated from the last 3–6 months:

Monthly Churn Rate = Churned MRR in period ÷ Starting MRR for period

Worked example:

• Starting MRR: €32,500
• Historical monthly churn rate: 2.1%
• Churned MRR forecast: €32,500 × 0.021 = €682.50

Why churn compounds: If you do not add any new customers, 2.1% monthly churn means you lose 22% of your current MRR over 12 months. (Compounded: €32,500 × (1 − 0.021)^12 = €25,412). This is why churn rate assumptions are the most sensitive variable in any SaaS forecast.

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Step 4: Add Expansion MRR

Expansion revenue comes from upgrades, seat additions, and usage-based revenue growth from existing customers.

Formula:

Expansion MRR = Starting MRR × Monthly Expansion Rate

If you have tracked expansion historically:

Monthly Expansion Rate = Expansion MRR ÷ Starting MRR of prior period

For many early-stage SaaS companies, expansion is modest (0.5–1.5% of MRR/month). For product-led growth businesses with usage-based pricing, expansion can exceed 3%/month.

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Worked Example 1: 12-Month Forecast (Conservative)

Starting point: €32,500 MRR in April 2026

Assumptions (base case):

• New MRR: €2,800/month (historical average)
• Monthly churn rate: 2.1%
• Monthly expansion rate: 0.8%
• Contraction MRR: €200/month (minor downgrades)

Month-by-month projection (first 6 months):

Month	Starting MRR	+ New	+ Expansion	− Churned	− Contraction	Ending MRR
May	€32,500	€2,800	€260	€683	€200	€34,677
Jun	€34,677	€2,800	€277	€728	€200	€36,826
Jul	€36,826	€2,800	€295	€773	€200	€38,948
Aug	€38,948	€2,800	€312	€818	€200	€41,042
Sep	€41,042	€2,800	€328	€862	€200	€43,108
Oct	€43,108	€2,800	€345	€906	€200	€45,147

12-month ending MRR (April 2027): approximately €54,200

ARR at April 2027: €54,200 × 12 = €650,400

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Worked Example 2: Scenario Range

A single-point forecast is not a forecast, it is a guess. Build three scenarios:

Assumptions table:

Scenario	New MRR	Churn Rate	Expansion Rate
Bear (pessimistic)	€1,500/month	3.0%	0.3%
Base (most likely)	€2,800/month	2.1%	0.8%
Bull (optimistic)	€4,500/month	1.5%	1.5%

12-month ending MRR projections:

Scenario	April 2027 MRR	ARR
Bear	€38,200	€458,400
Base	€54,200	€650,400
Bull	€78,500	€942,000

The range (€458k to €942k ARR) reflects genuine uncertainty. Present the base case with the range. If an investor or board member asks “what would have to be true for you to hit the bull case?”, you have a concrete answer: new MRR needs to improve 60%, and churn needs to drop 0.6 percentage points.

For scenario modeling methodology, see scenario modeling for bootstrappers.

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What to Exclude From Your Forecast

Include	Exclude
Recurring subscription revenue	One-time setup fees
Expansion from existing customers	Professional services / custom dev
MRR from trial conversions	Consulting revenue
Usage-based recurring components	One-time upsells with no recurring element

Non-recurring revenue should be modeled separately and clearly labeled as non-recurring. Mixing it into MRR inflates the recurring revenue picture and creates confusion in investor reporting.

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How to Defend Your Forecast

When presenting to investors, co-founders, or a board:

1. Ground assumptions in data, not aspirations

Every assumption should have a historical basis or a named change that would cause it to differ from history. “We assume 2.1% monthly churn because that is our average over the last 6 months” is defensible. “We assume 1% churn because we will improve the product” is not.

2. Show sensitivity to churn

Run a churn sensitivity table: what happens to 12-month MRR if churn is 1.5% vs 2.1% vs 3%? Showing you understand the sensitivity to the most important variable demonstrates analytical rigor.

3. Separate controllable from uncontrollable inputs

New MRR is largely controllable (you choose how much to spend on sales and marketing). Churn rate is semi-controllable (you can improve retention, but customers leave for reasons outside your control). Expansion is semi-controllable. Make this distinction explicit.

4. Reference your revenue growth formula

Being able to articulate: “our ending MRR = starting MRR + new MRR + expansion − churned MRR − contraction, and here are the assumptions behind each variable” signals that you understand your business model. See how to calculate revenue growth rate for the growth rate calculation that belongs alongside the forecast.

Track from Stripe. NoNoiseMetrics calculates your actual new MRR, expansion, churn, and contraction each month, exactly the inputs your revenue forecast formula needs. Try free

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Common Forecast Mistakes

Using revenue instead of MRR as the base

Annual plans paid upfront inflate cash-basis revenue in the month of payment. Use recognized MRR (normalized to monthly) for the starting point.

Assuming constant new MRR

New MRR tends to have seasonality and pipeline variability. Build in seasonal assumptions for slower months (August, December in most B2B markets).

Forgetting contraction MRR

Downgrade from a high-tier to a low-tier plan is not the same as churn, but it reduces MRR. If you have multiple plan tiers, model contraction separately from full churn.

Linear extrapolation without churn modeling

Simply multiplying current MRR by a growth rate ignores the compounding effect of churn. A 2% monthly churn rate means your existing base erodes 22% per year. New MRR must first replace lost MRR before it grows the base.

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FAQ

What is the revenue forecast formula for SaaS?

Ending MRR = Starting MRR + New MRR + Expansion MRR − Churned MRR − Contraction MRR. Apply this monthly for a 12-month forecast. Each month’s ending MRR becomes the next month’s starting MRR.

Which inputs do you need for a SaaS revenue forecast?

Five inputs: (1) current MRR (starting point), (2) expected new MRR per month, (3) monthly churn rate, (4) monthly expansion rate, and (5) expected contraction. All five should be based on historical averages, adjusted for any known changes in the business.

What should you exclude from the revenue forecast?

One-time fees, professional services, and non-recurring revenue. Keep the MRR forecast clean, recurring revenue only. Model non-recurring items separately and clearly label them as such.

Can you show a worked example of a SaaS revenue forecast?

Yes, see the examples above. With €32,500 starting MRR, €2,800 new MRR/month, 2.1% churn, and 0.8% expansion, the base case forecast reaches approximately €54,200 MRR by month 12. The bear case reaches €38,200 and the bull case reaches €78,500.

How often should you update the revenue forecast?

Update monthly as actuals come in. Compare actual results to the prior forecast, explain variances. A forecast that is never updated against actuals provides no learning value. Monthly reconciliation is the discipline that makes forecasting useful.

What is the most important variable in a SaaS revenue forecast?

Churn rate. A 1 percentage point change in monthly churn has a larger impact on 12-month MRR than a significant change in new MRR growth, because churn compounds on a growing base. Stress-test your forecast against a range of churn scenarios.

How do you forecast revenue for a brand new SaaS product?

With no historical data, use: (1) comparable company data for churn assumptions (2–3% monthly is typical for early-stage), (2) a bottom-up sales pipeline model for new MRR, and (3) a conservative expansion assumption (0.5%). Build wide confidence intervals and update aggressively as real data comes in.

What is the difference between ARR and MRR forecast?

ARR (Annual Recurring Revenue) = MRR × 12 at any given point. The MRR forecast gives you the monthly trajectory; multiply by 12 to get the ARR equivalent at any point. Most investors think in ARR for annual benchmarks, but the monthly model is more useful for operational decision-making.

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Related Reading

• SaaS Forecast Model: Forecast MRR with 3 Inputs, a simpler model for early-stage founders
• Scenario Modeling for Bootstrappers, how to build and stress-test scenario ranges
• How to Calculate Revenue Growth Rate, growth rate calculation alongside the forecast

External resources:

• OpenView Partners: SaaS growth benchmarks
• BVP Cloud Index: revenue growth analysis

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