The reference desk for payments & ecommerce

37 free tools for the money side of selling online: what payment processing really costs, whether your ads and margins actually work, the reference data payment developers look up every day — plus fast, private file utilities. No signup, no uploads, and every tool runs entirely in your browser.

  • Rates verified against official pricing pages (2026-08-02)
  • Embeddable on your own site
  • No accounts, no uploads

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All tools

Five families, one standard: accurate data, instant results, nothing stored. Payment fee calculators built on verified Stripe, PayPal, Square, and Shopify pricing; profitability math from ROAS breakevens to full unit economics; lookup references for test cards, decline codes, and merchant category codes; store operations templates; and image tools that convert and compress without uploading a single byte.

Payment fee calculators (5)

  • Stripe Fee Calculator

    Calculate exactly what Stripe charges on any payment — including international cards, currency conversion, and ACH — and what you should charge to receive a target amount.

  • PayPal Fee Calculator

    Calculate PayPal fees for every payment type — Checkout, card payments, QR, Goods & Services, Pay Later — including international surcharges, plus what to request to receive an exact amount.

  • Square Fee Calculator

    Work out exactly what Square charges per payment across online, in-person, and manually keyed transactions — and what to charge to net a target amount.

  • Credit Card Processing Fee Calculator

    Calculate the fee on any payment from your rate, or audit a monthly statement: your true effective rate and how it compares with Stripe, Square, and PayPal on the same volume.

  • Payment Processor Comparison

    Compare what Stripe, PayPal, Square, and Shopify Payments actually cost on your volume — enter your numbers, see ranked monthly and annual costs, effective rates, and where interchange-plus wins.

Profit & ad calculators (7)

  • ROAS Calculator

    Work out your return on ad spend from revenue and spend, or flip it around: the revenue you need to hit a target ROAS, and the max you can spend at your current revenue.

  • Breakeven ROAS Calculator

    Find the exact ROAS where your ads stop losing money, based on your AOV, product costs, shipping, and payment fees — with a profit table across ROAS levels.

  • Shopify Profit Calculator

    Calculate true profit per order on Shopify: plan-specific Shopify Payments rates, third-party gateway surcharges, product costs, shipping, and ad spend.

  • Break-Even Calculator

    Find how many units you must sell to cover fixed costs, your break-even revenue, and profit at different sales volumes — from price, variable cost, and fixed costs.

  • CAC & LTV Calculator

    Calculate customer acquisition cost, margin-adjusted lifetime value, your LTV:CAC ratio, and how many months it takes to pay back acquisition spend.

  • Discount Calculator

    Work out the final price after a discount (including stacked discounts), how much you save, or what percent off a sale price actually is.

  • Unit Economics Simulator

    Model your store's full unit economics: contribution margin per order, monthly P&L, break-even orders, and what a 0.5% fee change is really worth — shareable by URL.

Developer references & validators (6)

  • Stripe Test Cards

    Every Stripe test card number, searchable and one click to copy — successful payments, declines by code, fraud and Radar triggers, disputes, 3D Secure, international cards, and pm_ tokens.

  • Card Decline Code Lookup

    Look up any card decline code — Stripe codes, numeric issuer responses (05, 51…), AVS results, and Visa/Mastercard chargeback reasons — with who declined it, whether it is retryable, and what to do next.

  • MCC Code Lookup

    Search the full list of merchant category codes (MCCs) by number or description — the ISO 18245 codes card networks use to classify businesses for interchange, rewards, and risk.

  • IBAN Checker

    Validate any IBAN in your browser: country code, country-specific length, format, and the mod-97 checksum banks use. Nothing you type ever leaves the page.

  • Luhn Algorithm Checker

    Check whether any number passes the Luhn checksum, with a step-by-step visualization of exactly how the algorithm works — the check behind every card number.

  • UPC/EAN Check Digit Calculator

    Calculate or verify the check digit for any UPC-A, EAN-13, EAN-8, or GTIN-14 barcode number, with the GS1 mod-10 math shown step by step.

Store operations & templates (9)

  • SKU Generator

    Generate clean, consistent SKUs from a prefix, product attributes, and a numeric sequence — preview every combination and export as CSV. Runs entirely in your browser.

  • Chargeback Response Generator

    Pick the chargeback reason code and get a tailored evidence checklist plus a ready-to-edit rebuttal letter — for Visa and Mastercard disputes.

  • Invoice Generator

    Create a professional invoice in your browser and print or save it as a PDF — line items, tax, discounts, your logo. No account, no watermark, and your data never leaves the page.

  • Shipping Policy Generator

    Answer a few questions about your processing times, carriers, and international coverage — get a clear shipping policy you can paste into your store. No signup.

  • Inventory Turnover Calculator

    Calculate your inventory turnover ratio and days of inventory on hand from COGS and average inventory, with benchmarks for what a healthy ratio looks like.

  • EOQ Calculator

    Find the order quantity that minimizes total inventory cost — economic order quantity, orders per year, and an interactive curve of ordering vs holding costs.

  • Safety Stock Calculator

    Calculate safety stock from your demand variability, lead time, and target service level — including the combined formula for variable lead times.

  • Reorder Point Calculator

    Work out exactly when to reorder: average daily demand × lead time + safety stock, with the math shown and edge cases explained.

  • Sell-Through Rate Calculator

    Calculate sell-through rate from units sold and units received, with benchmarks by category and what to do when the rate is too low or too high.

Image & file tools (10)

  • HEIC to JPG Converter

    Convert iPhone HEIC photos to JPG right in your browser — bulk files, adjustable quality, and nothing ever uploaded to a server.

  • WebP to JPG Converter

    Convert WebP images to JPG instantly on your device — drag in multiple files, set quality, download. No upload, no signup, no watermark.

  • WebP to PNG Converter

    Convert WebP images to lossless PNG instantly in your browser — transparency preserved, multiple files at once, nothing uploaded anywhere.

  • AVIF to JPG Converter

    Convert AVIF images to JPG in one step, entirely in your browser — bulk conversion with adjustable quality and zero uploads.

  • PNG to JPG Converter

    Convert PNG images to smaller JPGs on your device — transparency flattened to white, quality slider, bulk files, no upload.

  • JPG to PNG Converter

    Convert JPG images to lossless PNG instantly in your browser — multiple files at once, nothing sent to any server.

  • PNG to WebP Converter

    Convert PNGs to modern WebP for dramatically smaller web images — adjustable quality, bulk files, all on your device.

  • JPG to WebP Converter

    Convert JPGs to WebP for faster pages and smaller files — quality slider, bulk conversion, zero uploads.

  • JPEG Compressor

    Compress JPEG photos to a fraction of their size with a quality slider and instant before/after readouts — all on your device, never uploaded.

  • PNG Compressor

    Shrink PNG files with 256-color quantization (the pngquant technique) right in your browser — typically 50–70% smaller with transparency preserved.

Why merchants and developers use StopByte

Verified, dated data

Fee calculators are only as good as their rates. Ours are checked by hand against stripe.com, paypal.com, squareup.com, and shopify.com pricing pages — and every tool shows the date it was last verified, so you never trust stale numbers silently.

Private by architecture

Nothing you type or upload reaches a server, because there is no server — every calculation, lookup, and image conversion runs in your browser. That makes the tools fast, safe for sensitive numbers, and usable offline once loaded.

Built by people who ship payments

StopByte has served developers since 2016 — the original Q&A forum lives on in our archive. Today the focus is the practical money math of running an online store, from the first Stripe charge to routing volume across processors.

Latest guides

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Questions, answered

Are these tools really free?

Yes — every calculator, reference, and converter on StopByte is free, with no account, no usage limits, and no locked features. Most tools are also embeddable on your own site.

Is my data stored anywhere?

Your tool data is not. Every tool runs entirely in your browser — calculator inputs, invoices, and images are never uploaded or stored; shareable tools encode state in the URL you copy, not on a server. We use Google Analytics for aggregate page statistics only, and it never sees what you type into the tools.

How do you keep fee data accurate?

Every rate we publish — Stripe, PayPal, Square, Shopify Payments — is manually verified against the provider’s official pricing page, and each tool shows its verification date. If a provider changes pricing, the date tells you exactly how fresh our numbers are.

What happened to the StopByte forum?

StopByte ran a developer Q&A forum from 2016. All 225 threads are preserved read-only at their original addresses in the forum archive, while the site now focuses on payments and ecommerce tools.