Why I Built Senaro™: I Had No Idea How to Efficiently Pay Off $80K Across 7 Credit Cards

$80K across 7 credit cards. That was my life for a while. COVID, family health stuff, supporting people. Never missed a payment in nearly two decades. Not once. But I was paying over $1,400/month in interest alone — and just throwing random extra payments at whatever card felt right. The balances barely moved.

Why Paying Off Multiple Credit Cards Is So Hard

I had no way to see what was actually happening with my money across all 7 cards. Which card was costing me the most in interest? Where should that extra $50 go — toward the highest-APR card or the smallest balance? Was I even making progress, or just treading water?

So I did what any developer does — I opened Excel.

At first it worked. I used weighted average APR to build a payment schedule, and for the first time I felt like I had some control. But then the payments started looking off. I tried building better prioritization logic, cross-referenced bank statements — and before long the formulas were out of control. No version history, no way to track what I changed.

I still love Excel for quick math. But 7 cards with multiple APRs and payment allocation rules? I needed something I could maintain. So I switched to .NET with real dashboards and charts.

The Moment Everything Clicked

I built a chart that showed my projected balances over time with just minimum payments. My total balance was slowly going down. But the cash advance balance? That line went UP.

I was making the minimum payment every month and the balance was still growing.

That was my cash advance balance — 29.99% APR, no grace period, generating $125/month in interest on its own. My minimum payment was barely covering it. Seeing that visually was a gut punch.

Then I added $50 extra per month in the model. The difference in total interest was massive. $100 extra — even bigger. It wasn't just "pay more, owe less" which is obvious. It was seeing exactly how many months and how many thousands in interest that small amount actually saved across all 7 cards.

The Real Cost of Cash Advances at 29.99% APR

I didn't fully understand how cash advances worked until I went back through my bank statements. I had $9,800 in cash advance balances across two cards — one at 29.99% APR ($5K) and another at 24.99% ($4.8K). But I hadn't taken a cash advance in over two years. So how did the balances grow?

I pulled statements from a year back, two years back. The balances were lower. So I dug into the math — how the CARD Act allocates payments, how interest compounds daily with no grace period on cash advances, what happens if I just keep paying minimums. The two cards combined were generating about $245 in interest every single month just on the cash advance portions. My minimum payments barely covered that. A few dollars toward the actual balance. Some months, not even that.

I ran it forward. Without action, the $9,800 would have taken over 25 years to pay off — and I would have paid over $38,000 total. Nearly 4x what I originally borrowed. I needed to stop the bleeding.

So I did. I got a bonus, a tax return, and I threw everything at the cash advances first. Closed out the majority in a few months. The rest over the next year. That was the first time I made a debt decision based on actual math instead of gut feeling.

Under the CARD Act, minimum payments go to the lowest-APR balance first. So my purchases at lower APRs were getting paid down while the cash advance balances just sat there growing. Most credit card payoff calculators don't account for this — they treat each card as one APR. That's why my numbers never added up.

From Excel to .NET to the Web

I went back and forth between Excel and .NET like three or four times before I committed. The .NET app was my personal tool — it's what helped me figure out my own debt. I could see month-by-month where every dollar was going: principal vs interest, card by card.

Eventually I rebuilt the calculator as a static HTML site and shipped it. No framework, no backend. Just HTML, CSS, and JavaScript. I wanted it to be something anyone could use without signing up or handing over their financial data.

What Every Credit Card Debt Calculator Gets Wrong

I even tried Mint for a while to get a handle on things. It was great for seeing where money was going, but it didn't help me figure out how to pay it off faster. And every credit card debt calculator I tried — NerdWallet, Bankrate, Credit Karma — treated each card as a single APR. None of them addressed the things that were actually costing me money:

• How even small extra payments ($10, $50, $100) change your total interest and payoff date — not in theory, but with your exact numbers across multiple cards
• Separate Purchase APR from Cash Advance APR per card — the thing that was invisibly growing on me
• Avalanche vs Snowball comparison side by side — so you can see the actual dollar difference, not just read about which is "better"
• Month-by-month breakdown showing where every dollar goes — principal vs interest, card by card

Senaro™'s credit card payoff calculator does all of this. No signup, no bank linking. It runs in your browser and saves to localStorage.

How the Debt Avalanche Method Saved Me Thousands

Before I had the tool, I was throwing extra payments at random cards. Sometimes the highest balance. Sometimes the one I used most recently. No system.

Once I could see the numbers, I targeted the 29.99% cash advance first — the worst rate in my stack. Tax return came in, a bonus. I put everything on that one card. $1,500 one month. $4,350 the next.

The cash advance went from $5,000 to zero in about two months. That was the moment I knew this approach worked.

Then I moved to the next highest rate. That's the debt avalanche method — you don't pay off the smallest card first for a quick win, you go after the card that's costing you the most in interest. For me, that shift alone cut about $50-100/month in interest compared to my old approach of guessing. Over a year, that adds up.

The snowball method — smallest balance first — works too. Some people need the motivation of watching a card hit zero. (Investopedia has a good breakdown of how the two compare.) Senaro™ shows you both strategies side by side with your actual numbers, so you can see the dollar difference and pick what works for you.

In this example: $32,500 across 5 cards (12.99%–29.99% APR), with $100/month extra toward payoff.

The Debt Payoff Mistakes I Made Before I Had Visibility

Here's what I was doing wrong before I had the numbers in front of me:

• Paying extra on random cards — Some months I'd throw $200 at my biggest balance, other months the card I just used. No math behind it. Just whatever felt right.
• Assuming minimum payments attack the highest APR first — I knew about the 29.99% cash advance rate. I just assumed the bank would apply my payment there first. Logically, that's what you'd expect, right? They don't. Under the CARD Act, minimums go to the lowest rate first. I didn't check until the balance stopped moving.
• Only looking at total balance — My total was going down, so I thought I was winning. I wasn't looking at the breakdown. One balance was literally growing while I made payments.
• Not modeling extra payments — I assumed an extra $50 wouldn't matter on $80K of debt. It did. On a 29.99% card, $50 extra per month cut years off the payoff.

Every one of these cost me real money. The common thread? I couldn't see what was happening. Flying blind with five-figure debt.

What Changed For Me

Once I could actually see the math, I stopped guessing. I targeted the right cards in the right order using the debt avalanche strategy. I could see exactly what an extra payment would save me — not someday, but in specific months and specific dollars.

I've paid off about half my balance since then.

The tool didn't change my income or my situation. It changed my visibility. And that changed my behavior.

What's Next

Right now Senaro™ is a free calculator. Pure math, runs locally. The plan is to add AI-powered optimization down the road — smarter recommendations, what-if scenarios, that kind of thing. The core calculator will always be free. No catch.