SysAdmin tools
19 tools that run in your browser. Nothing to install, nothing uploaded.
Permission maths and system-administration references — umask, chmod, and the lookups that come up when you are elbow-deep in a Unix box. Pure calculations that run in your browser, and precise about the parts that bite: a bare three-digit chmod silently clears the special bits, a world-writable mask is flagged, and every result is spelled out in both octal and symbolic form so there is no ambiguity about what it grants.
- Backup Window CalculatorWork out whether a backup job finishes before the window closes.
- Bandwidth Cost CalculatorEstimate what outbound data transfer costs you each month.
- Bandwidth per User PlannerSize an internet connection on the people using it at once.
- chmod / Permission CalculatorConvert Unix file permissions between octal, symbolic and plain English.
- Cloud vs On-Prem Storage CostFind the month a NAS stops costing more than a cloud subscription.
- Crontab GeneratorBuild a cron schedule and read it back in English — including the day-field trap.
- Downtime Cost CalculatorPut a number on an hour of outage so the argument stops being about opinion.
- Ethernet Cable ReferenceWhat each Ethernet category actually does, and how far it does it.
- PoE Power Budget PlannerCheck whether a switch can really power everything you plug into it.
- Rack Unit (U) Space PlannerCount the rack units your kit needs, including the gaps you forgot.
- RAID Capacity CalculatorUsable capacity, fault tolerance, and the rebuild risk nobody prints.
- RAID Rebuild Time EstimatorEstimate how long an array runs degraded after a drive fails.
- Server RAM CalculatorWork out how much memory a host needs before you order it.
- SLA Comparison TableWhat each availability target actually allows, from 99% to five nines.
- Uptime / SLA CalculatorTurn an uptime percentage into the downtime it actually permits.
- Storage Unit ConverterConvert between TB and TiB, GB and GiB, and see why they differ.
- umask CalculatorSee exactly what permissions new files and directories get from a umask.
- UPS Runtime EstimatorEstimate how long a UPS really holds a load, batteries and losses included.
- Wake-on-LAN Packet BuilderBuild a Wake-on-LAN magic packet — and see why a browser cannot send it.
Permission maths where a wrong digit is a security incident
Unix permissions look simple until the special bits get involved, and the failure mode is silent. Setting a mode with three octal digits clears the setuid, setgid and sticky bits without warning, so a chmod 755 on a directory that relied on setgid quietly changes who owns new files inside it. A world-writable bit on a directory without the sticky bit lets any user delete another user’s files. The calculators here spell out every result in octal, in symbolic form and in plain English at once, and flag the modes that grant more than people usually intend.
The cron field that catches everyone
Cron’s day-of-month and day-of-week fields do not combine the way the rest of the expression does. When both are set to something other than the wildcard, cron runs the job if either matches, not if both do — so an entry meant to fire on the first of the month only when it falls on a Monday actually fires on every first and every Monday. The generator reads any schedule back to you in English precisely so this is visible before the job is installed rather than after somebody notices it ran four times as often as intended.
Capacity, redundancy and the number vendors do not print
RAID arithmetic gives you usable capacity and how many drives can fail, and both are easy to look up. The figure that matters and is rarely stated is rebuild exposure: replacing a failed drive in a large array means reading every remaining drive completely, for hours or days, at exactly the moment the array has no redundancy left. On large consumer drives the probability of hitting an unrecoverable read error during that rebuild stops being negligible, which is the real argument for double-parity layouts on big arrays — and the reason RAID has never been a backup.
What an uptime percentage actually permits
Service level agreements are quoted in nines because the percentages sound reassuring and the durations do not. Three nines is 99.9%, which permits about 8 hours and 45 minutes of downtime a year, or roughly 43 minutes a month. Four nines allows about 52 minutes a year. The uptime calculator converts a percentage into the time it actually buys, over a day, a month and a year, which is the form the number needs to be in before you can judge whether a contract means anything or whether a single bad afternoon has already spent the annual budget.
Frequently asked questions
Why did chmod 755 change more than I expected?
Because a three-digit mode sets all four octal digits, with the first defaulting to zero — which clears the setuid, setgid and sticky bits without saying so. On a shared directory that relied on setgid to keep group ownership consistent for new files, that is a silent behaviour change nobody notices until the permissions on new files start looking wrong. Use a four-digit mode, or symbolic notation, when special bits are in play.
Why does my cron job run more often than the schedule says?
Almost certainly the day-of-month and day-of-week interaction. When both fields are set to something other than the wildcard, cron treats them as OR rather than AND — so an entry intended for the first of the month when it falls on a Monday runs on every first and on every Monday. Reading a schedule back in plain English before installing it is the reliable way to catch this, because the expression itself looks entirely correct.
Is RAID a backup?
No, and treating it as one is how people lose data. RAID protects against a drive failing; it does nothing about deletion, ransomware, corruption, a failed controller, theft or fire, all of which are faithfully written to every disk in the array at once. The rebuild window is its own risk too — replacing a drive means reading every remaining one completely while the array has no redundancy left, which on large drives is precisely when a second failure surfaces.
How much downtime does 99.9% uptime actually allow?
About 8 hours and 45 minutes a year, or roughly 43 minutes a month. Four nines, 99.99%, allows about 52 minutes a year. Percentages are quoted because they sound reassuring in a way the equivalent durations do not — and converting one into the other is usually the moment somebody realises a single bad afternoon has already spent the year’s allowance.