Runway Math for Your Side Project (or Your SaaS)

William Willison — Sat, 31 Oct 2026 05:08:41 +0000

You don’t need an accountant to answer the one question every cash-strapped project has to face: how many months before the money runs out? That number is your runway, and it’s one division away from your bank balance.

The three numbers that matter

Net cash flow = inflows − outflows, per month. For a side project: revenue in, hosting + tooling + time-equivalent out.
Burn = what happens when outflows exceed inflows. That’s the deadline-setter, not the average.
Runway = current cash ÷ burn. Not “how long the project lasts” — how long it lasts at the current burn rate.

A worked example
A small SaaS brings in $8,000/month and spends $6,200 — net +$1,800, which feels safe until revenue dips 30% for a quarter: $5,600 in, $6,200 out, now burning $600/month. With $10,000 in the bank, runway at that burn is $10,000 ÷ $600 ≈ 16.7 months. Plenty — until you add the $20,000 equipment purchase you’ve been deferring, which is 33 months of that burn, and the cushion is gone.

The habit that matters: re-enter these numbers monthly from receipts, not from the plan. A forecast is a promise; a receipt is a fact.

I keep the inflow/outflow/runway math in one free screen (no signup): the cash flow calculator at CalculatorPro (calculatorpro.org/finance/cash-flow). Type in the two sides and it returns the net flow, your ending balance, and the runway number to argue with.

Informational only — not financial advice.

Stop Rewriting Simple Calculation Logic — There’s a Smarter Way

William Willison — Tue, 05 May 2026 06:47:40 +0000

If you’ve built even a few projects, you’ve probably written the same calculation logic more times than you’d like to admit.

Percentages. Ratios. EMI formulas. Unit conversions. Basic arithmetic wrapped in utility functions.

At first, it feels fine. You write a helper function, test it quickly, and move on.

But over time, it becomes repetitive.

The Real Issue Isn’t Complexity

Let’s be honest — most of these calculations aren’t hard.

The problem is:

rewriting the same logic again and again
verifying correctness every time
breaking your flow just to check a formula

Even something as simple as a percentage increase can pull you out of your coding context.

You stop → search → calculate → return → resume coding.

That context switching adds up.

When External Tools Actually Make Sense

For production code, sure — you’ll implement your own logic.

But for:

quick validation
prototyping
debugging
internal tools

Using external calculators can save time.

Instead of writing temporary logic or guessing formulas, you can validate results instantly.

If you want a practical look at how these tools simplify everyday calculations, this
👉 quick breakdown of calculator workflows.

Reduce Friction in Your Workflow

Think about how often you:

test math-heavy logic
double-check financial calculations
validate outputs manually

Now imagine skipping that step.

Instead of writing a quick script or console log, you just plug values into a tool and confirm the result.

For example, using
👉 a centralized set of online calculators
lets you test and verify values without writing extra code.

It’s especially useful when you’re:

building MVPs
debugging edge cases
validating user inputs
It’s Not About Avoiding Code

This isn’t about replacing your logic.

It’s about:

working faster
reducing repetition
minimizing small mistakes

You still write clean, reliable code where it matters. But you avoid wasting time on things that don’t need to be rebuilt.

Where This Helps Most

From experience, this approach is useful in:

Rapid Prototyping

You don’t want to build everything from scratch just to test an idea.

Debugging

Quickly confirm if your calculations are correct.

Learning

Understand formulas without getting stuck in manual math.

Internal Tools

Speed matters more than perfect implementation.

Final Thoughts

As developers, we focus a lot on performance, architecture, and clean code.

But improving your workflow is just as important.

If you can remove even small interruptions from your process, it makes a noticeable difference over time.

Sometimes the smartest move isn’t writing more code — it’s knowing when not to.

DEV Community: William Willison

Runway Math for Your Side Project (or Your SaaS)

Stop Rewriting Simple Calculation Logic — There’s a Smarter Way