Networking & website diagnostics
Find where a connection is failing
Start with the IP range and port, then separate TLS certificate problems from HTTP errors.
Start with your task
What are you working on?
Pick a subject and follow the most useful starting points.
Time & date
Work with dates, clocks and durations
Convert a time zone, compare calendar dates, or translate seconds to readable units.
Money & planning
Model financial choices with context
Estimate payments and savings while understanding which assumptions affect the result.
Developers & data
Inspect the result before acting
Decode HTTP responses, inspect request timing, and validate data in your browser.
New to the library
Recently Added
A small selection of the latest tools, without another long directory on this page.
Developer
HTTP Status Code Reference
Look up HTTP response codes, understand redirects and authentication errors, and troubleshoot client, server, and gateway failures.
Health
Baby & Toddler Feeding Scheduler
Create an age-based feeding timeline from birth through 24 months with options for breast milk, formula, combination feeding, solids, and daily schedule timing.
Math
Lottery Number Generator
Generate random lottery number combinations with common game presets, custom ranges, exclusions, multiple lines, and browser-based cryptographic randomness.
Finance
Fix & Flip Profit Calculator
Estimate purchase, renovation, financing, holding, and selling costs with projected profit, ROI, break-even price, and target purchase price.
A few useful starting points
Featured Tools
Quick access to popular kinds of calculations and diagnostics.
Time & Date
Time Zone Converter
Compare multiple world time zones at a glance with DST-aware IANA time zone conversion, meeting overlap highlighting, and 12/24-hour formats.
Network
Bandwidth Calculator
Free bandwidth calculator for transfer time, required bandwidth, and transferable file size. Supports Mbps, Gbps, MB/s, GB, GiB and practical efficiency estimates.
Developer
IP Subnet Calculator
Free IPv4 and IPv6 subnet calculator. Calculate network address, broadcast, host range, subnet mask, usable hosts and IPv6 prefix information.
Math
Volume Calculator
Calculate the volume of cubes, spheres, cylinders, and cones instantly with our free browser-based tool.
Time & Date
Days Between Dates Calculator
Free days-between-dates calculator for elapsed or inclusive days, weeks, weekdays, weekends, leap days, and calendar span.
Finance
Tip Calculator
Free online tip calculator. Calculate gratuity, tax, service charge, total bill, and per-person amount. Split restaurant bills and choose common tip percentages instantly.
Learn the why
Practical explanations behind the tools
Read a worked explanation when a quick result isn't enough.
CIDR and subnet masks explained
Learn what /24, /31 and /32 mean, why usable-address counts vary, and when the host-range calculation needs context.
Try the IP Subnet CalculatorTCP and UDP: lookup versus reachability
Understand why a service's usual port does not prove a host is listening, and why silence from UDP cannot be treated as a definite failure.
Look up common portsWhy SSL/TLS certificate checks fail
Separate expiration, hostname mismatches, missing chain certificates and connection issues before deciding what to fix.
Check a public certificateBeyond the number
Useful answers include their assumptions.
A subnet calculator doesn't test your firewall, and a financial estimate doesn't predict future fees. Our tools link to explanations so you can tell what a result means—and when more investigation is needed.