Why We Built a Pension Calculator That Actually Makes Sense

I'll never forget the moment that led to BishBashDosh.

It was 2023, and I'd just received my first payslip after opting into my company's pension scheme. Something felt... off. The numbers didn't quite add up. My take-home pay had dropped, but not by the amount I expected.

So I did what any reasonable person would do: I opened a spreadsheet.

Four hours later, I was deep in HMRC guidance documents, trying to understand how salary sacrifice actually worked. The more I learned, the more frustrated I became.

Not because pensions are complicated (they are), but because the existing tools were useless.

#The Problem with Existing Pension Calculators

Most pension calculators fall into two camps:

#The Oversimplified

These tools ask for your salary, give you a percentage, and spit out a number. They ignore:

• Employer contribution matching policies
• The impact of salary sacrifice on National Insurance
• Tax relief at different income levels
• The £100k-£125k personal allowance taper trap
• Year-to-date accumulation for the annual allowance

Real-world example: I found calculators telling a £55,000 earner to contribute 5% while completely missing that pushing to 8% would save them an extra £600/year in NI alone.

#The Overcomplicated

Then there are the "professional" tools that require you to:

• Know your marginal tax rate (most people don't)
• Understand taper calculations
• Input 15 different variables
• Have a finance degree to interpret the results

Neither approach helps the person who just wants to know: "How much should I put in my pension this year?"

#The £10,000 Question

Here's what really drove me to build BishBashDosh:

A colleague earning £110,000 was contributing the bare minimum to their pension. When I ran the numbers, they were losing over £10,000 per year in potential tax savings.

Ten. Thousand. Pounds.

Not because they were lazy or didn't care about their future. They simply didn't know. The existing tools either:

• Told them to "contribute as much as you can afford" (not helpful)
• Showed calculations they couldn't understand (not actionable)
• Ignored salary sacrifice entirely (missing the biggest benefit)

#Building Something Better

I wanted to build a calculator that:

1. Shows you the real numbers: Your actual take-home pay after salary sacrifice, including the NI savings most calculators miss

2. Handles the edge cases: The £100k-£125k trap where you lose £1 of personal allowance for every £2 earned. The £260k pension allowance taper. The interaction between child benefit and salary sacrifice.

3. Optimizes automatically: Not just "here are the numbers" but "here's exactly what you should do to pay the least tax legally possible"

4. Tracks year-to-date: Because your circumstances change throughout the tax year, and your April optimization might not work in November

#The Technical Challenges

Building an accurate UK tax calculator is harder than it sounds.

#Challenge 1: The Personal Allowance Taper

Between £100,000 and £125,140, you lose £1 of personal allowance for every £2 you earn over £100k. This creates an effective marginal tax rate of 60%.

But here's the thing: salary sacrifice reduces your adjusted net income before this calculation. So contributing an extra £1,000 to your pension doesn't just save you £400 in tax (40% rate) — it saves you £600 because you also preserve £1,000 of personal allowance.

Most calculators miss this completely.

#Challenge 2: Employer Contributions

Some employers match your contributions. Some contribute a fixed percentage. Some have tiered structures. Some only match on certain salary bands.

We needed to handle all of these scenarios while showing users exactly how much "free money" they were leaving on the table.

#Challenge 3: Year-to-Date Tracking

The pension annual allowance is £60,000. But what if you contributed £20,000 in April, then got a bonus in September?

Our calculator needed to track YTD contributions and warn you before you accidentally triggered:

• Annual allowance excess charges
• Reduced annual allowances for high earners
• Lifetime allowance issues (before it was abolished)

#What We Learned from Early Users

When we first launched, we made some assumptions that turned out to be wrong:

Assumption: People want detailed breakdowns of every tax calculation Reality: People want the bottom line and one clear action to take

So we redesigned the interface to show:

• Your current take-home pay
• Your optimized take-home pay
• Exactly what to change in your payroll system
• The £ amount you're currently leaving on the table

Assumption: Advanced users want every possible customization option Reality: Even advanced users want the default to "just work"

We moved 80% of the options into an "Advanced" section and made the default settings accurate for 95% of users.

Assumption: People understand how salary sacrifice works Reality: Even HR professionals sometimes get this wrong

We added explanations, examples, and a "How This Works" guide that breaks down each calculation in plain English.

#The Edge Case That Almost Broke Us

In March 2024, we got an email from a user:

"Your calculator is wrong. I earn £130k, contribute £30k to my pension, but you're showing the wrong tax amount."

After digging in, we discovered they had:

• Base salary of £100k
• Annual bonus of £30k
• Employer pension contribution of 5% on base only
• A previous pension from another job with £15k already contributed this tax year
• A child benefit claim being reduced by the high income charge

Our calculator was handling each piece correctly but didn't account for the interaction between all five factors simultaneously.

We spent two weeks rebuilding the calculation engine to handle multi-source income, multiple pension pots, and complex benefit interactions.

That one user's edge case made the calculator better for everyone.

#Why Accuracy Matters

When you're building a tool that tells people how to structure their finances, accuracy isn't optional.

We've spent hundreds of hours:

• Reading HMRC guidance documents
• Testing edge cases
• Verifying calculations against real payslips
• Updating for every Budget announcement
• Consulting with accountants and IFAs

Because getting this wrong doesn't just mean showing the wrong number — it means someone might:

• Underpay into their pension and lose tax relief
• Overpay and trigger allowance charges
• Miss out on employer contributions
• Structure their salary inefficiently

The stakes are high. People trust us with their financial decisions.

#What We're Working on Next

The calculator today is vastly better than the spreadsheet I built in 2023, but we're not done.

Coming soon:

• Tax year forecasting: Project your full year based on YTD data
• Scenario comparison: Compare multiple salary sacrifice strategies side-by-side
• Budget integration: Automatic updates when tax rules change
• Family optimization: Show how your pension contributions affect household benefits
• Historical analysis: See how your strategy has performed over time

#The Real Reason We Built This

Here's what it comes down to:

The people who need salary sacrifice optimization most are often those who least understand it.

Someone earning £50k might not realize that £2,000 more into their pension only costs them £1,140 in take-home pay (thanks to tax relief and NI savings).

Someone earning £110k might not know they're in the worst tax trap in the UK system.

Someone with kids might not realize their pension contributions can eliminate the child benefit charge entirely.

These aren't edge cases. These are thousands of UK workers leaving money on the table every single month because the tools to help them either don't exist or are too complicated to use.

We built BishBashDosh to fix that.

#Try It Yourself

If you've read this far, you probably fall into one of two camps:

1. You're optimizing your pension already and want to verify you're doing it right
2. You suspect you could be doing better but aren't sure where to start

Either way, run your numbers through BishBashDosh. It's free, it's accurate, and it might just find you a few thousand pounds you didn't know you were losing.

And if you find an edge case we missed? Email me. Seriously. That's how we make this better.

Elliot Taylor Founder, BishBashDosh

Have questions about salary sacrifice or how the calculator works? Drop us a message through our support chat — we love talking about UK tax optimization (yes, really).