MundoKitSmall, honest tools for new questions
51 free calculators, converters and generators. Each one shows its formula and sources, runs in your browser, and never asks you to sign up.
AI & LLM tools
Estimate LLM API bills and prompt-caching savings, size GPU memory for local models, convert JSON to TOON, and publish llms.txt and AI-crawler robots.txt rules.
- AI agent cost calculatorEstimate growing conversation costs with a per-turn ledger, retries and reasoning assumptions.
- AI crawler robots.txt generatorChoose AI crawl preferences, merge robots.txt, and test paths with RFC matching rules.
- Context window calculatorCheck instructions, schemas, documents, history and output reserves against model limits.
- JSON to TOON converterConvert JSON and TOON locally, verify a round trip, and compare text sizes.
- LLM API cost calculatorEstimate token costs per request, day and month with editable prices and model comparisons.
- LLM VRAM calculatorEstimate local LLM weight and KV-cache memory, GPU fit, and maximum context.
- llms.txt generatorBuild a Markdown guide to your site, validate its structure, and download llms.txt.
- Long-context pricing calculatorCompare costs around a prompt threshold and budget a distribution of request sizes.
- Prompt caching savings calculatorCompare cache writes and reads with ordinary input billing, and find the reuse break-even point.
Money & tax
Estimators for the 2025–2028 US deductions (overtime, tips, seniors, car-loan interest, SALT), 2026 contribution limits and UK student loans. General information with IRS and GOV.UK sources, not financial advice.
- 2026 401(k) contribution limit calculatorFind employee deferral room, catch-up eligibility and the paycheck amount needed to reach your limit.
- Car loan interest deduction calculatorCheck eligibility, apply the income phase-out and optionally estimate yearly loan interest.
- Excess Social Security tax calculatorEstimate the credit from multiple employers and separate withholding errors an employer must correct.
- No tax on overtime calculatorEstimate the qualified overtime deduction, income phase-out and optional federal tax savings.
- No tax on tips calculatorEstimate your tips deduction after business net-income limits, the return cap and income phase-out.
- SALT cap calculatorFind your income-adjusted SALT cap, allowed itemized deduction and taxes excluded by the cap.
- 2026 senior deduction calculatorEstimate the enhanced senior deduction and the separate 2026 age-based standard deduction addition.
- Trump account contribution room calculatorCheck 2026–2027 contribution room by source and illustrate growth at a return you choose.
- UK student loan repayment calculatorEstimate 2026/27 payroll deductions for Plans 1, 2, 4, 5 and postgraduate loans.
Creators & media
Static QR codes, post length checks for X, Bluesky, Threads and more, subtitle timing fixes, video size math, and image compression and metadata removal that never upload your files.
- Compress image to a target KB sizeCompress JPG, PNG and WebP locally with a target size, quality search and automatic resizing.
- EXIF metadata remover for JPEG and PNGRemove JPEG and PNG metadata locally without re-encoding image data. Inspect EXIF and keep ICC optionally.
- QR code generatorCreate static QR codes for text, links, Wi-Fi and contacts. Download SVG or PNG locally.
- Social post character counterCompare a draft with eight platform limits, count Unicode text and split numbered threads.
- Subtitle time shifterShift SRT or WebVTT cues, fix drift with two anchors, and convert subtitle formats locally.
- Video file size calculatorEstimate video size from bitrate or find the video bitrate that fits a target upload size.
Developer & IT
Developer utilities that keep data on your machine: JWT decoding, UUIDv7 and ULID, epoch timestamps, cron schedules, IPv4/IPv6 subnets, OKLCH colors, CSS clamp() and SLA budgets.
- CIDR subnet calculatorFind network ranges, masks, address counts, equal subnets, and IP membership for IPv4 or IPv6.
- Cron expression explainerExplain five-field cron syntax and preview upcoming run times in an IANA time zone.
- CSS clamp generatorGenerate fluid font sizes with px or rem endpoints and inspect the interpolation math.
- JWT DecoderDecode JWT claims locally and inspect expiration dates without uploading a token.
- OKLCH color converterConvert CSS colors, preview sRGB fallbacks and check WCAG text contrast.
- SLA uptime & downtime calculatorTurn an availability target like 99.9% into allowed downtime per day, month and year.
- Unix Timestamp ConverterConvert Unix timestamps and ISO dates in batches with exact nanosecond arithmetic.
- UUIDv7 Generator and UUID / ULID DecoderGenerate secure random UUIDv7 or ULID batches and inspect embedded timestamps.
EV, energy & home
EV charging cost and time, and electric-versus-gas ownership cost after the end of the US federal EV credit, with DOE and EPA methods.
Travel, visas & customs
Entry rules, visa-free stays, e-authorizations, tax-free shopping refunds, duty-free allowances and travel practicalities, each checked against official government sources with the date we checked.
- China 24-hour transit checkerCheck a short China transit and whether leaving the restricted area requires a temporary entry permit.
- China 240-hour visa-free transit checkerCheck nationality, route, entry ports, permitted areas and the next-midnight transit deadline.
- China accommodation registration deadlineCheck who handles lodging registration and create a private-stay deadline reminder in Beijing time.
- China cash declaration checkerCheck RMB cash caps and foreign-currency declaration or exit-permit thresholds for one traveler.
- China customs allowance calculatorCheck return-shopping value, tobacco, vaping and alcohol limits by residence and arrival route.
- China foreign card payment fee calculatorEstimate wallet and issuer fees per purchase, with editable rates and confirmed promotion overrides.
- China–Malaysia visa-free stay calculatorCount Malaysian visitors’ prior visa-free days in China and calculate the next stay allowance.
- China tourist tax refund calculatorEstimate net departure refunds and check purchase dates, eligibility and Shanghai instant refunds.
- China visa-free entry checkerCheck ordinary-passport lists, waiver expiry dates and the unilateral 30-day departure deadline.
- Hainan duty-free quota calculatorTrack your annual RMB 100,000 quota and check category quantities across shops on the same trip.
- Hainan visa-free entry checkerCheck Hainan’s regional passport list, arrival route, travel area and 30-day stay deadline.
- Japan tax-free shopping calculatorCompare old checkout savings with the new refund system, including fees and the 90-day deadline.
- Korea tax refund calculatorCheck receipt and trip limits, then estimate a VAT refund with an editable example agent fee table.
- Power bank Wh calculatorConvert mAh and nominal voltage to Wh, then screen capacity and quantity for flights.
- Schengen 90/180-day calculatorCount rolling Schengen stay days and estimate the latest exit for a new trip.
Country tax & salary
Take-home pay, income tax and payroll calculators for specific countries, built from the official tax authority tables for the current tax year. General estimates, not tax advice.