WordPress capacity estimator
Work the sizing question backwards: name a server's RAM and cores, describe the sites, and see the band of WordPress or WooCommerce installs it carries, with the resource that hits the wall first named in the result.
Capacity is the server-size question read backwards: not how big a box a set of sites needs, but how many sites a box holds. Enter the RAM and core count, describe the sites, and get the band each resource line supports, the smaller of the two, and which one is the wall.
By Sam Moreton
Researched
Calculator
Assumptions
- Per-site baselines, the same planning baseline the server-size calculator declares (web server, PHP pool floor, database, object cache): WordPress 0.8-1.2 GB, WooCommerce 1.0-1.6 GB.
- Database working set above the baselines, taken off the usable band once per server: 0.25-0.5 GB at medium intensity, 0.75-1.5 GB at high.
- Admin and editor activity above the baselines, also once per server: 0.1-0.25 GB at medium, 0.25-0.5 GB at high.
- CPU: one core per 2 GB of planned RAM, the same planning line the server-size calculator declares, read backwards as cores x 2 GB.
- The operating system and its services reserve 0.5-1 GB of the server's RAM before site planning starts.
- RAM kept free for spikes: none plans on every usable gigabyte, modest keeps 25% free, large keeps 50% free. The reserve is RAM only: the core count is the core count.
Formula vv0.1 / last reviewed 2026-10-10
Every server-size question runs in two directions. The common one starts with the sites and asks how much box to buy. This tool starts with the box: name its RAM and cores, describe the sites it carries, and it answers with how many WordPress or WooCommerce installs fit, and which resource runs out first.
Who this is for
Agencies packing client sites onto shared servers, and anyone who inherited a box and wants to know whether three more sites fit before the next sales call. If you maintain a roster of similar sites, this is the question you ask before every new signup.
Reading the result
The engine plans each site at a per-site baseline, reserves a slice for the operating system, takes the shared database and admin load off the top once, and applies the spike reserve you choose. Then it counts sites twice, once against RAM and once against the cores. The answer is the smaller count, and the binding resource is named, because the two walls behave very differently when you upgrade.
The CPU wall is the one people miss. The planning line gives each core 2 GB of sites to carry, so a small-core box tops out well below its advertised RAM. Buying the bigger memory tier on a CPU-bound server changes nothing, and the calculator shows you that before you pay for it.
What the band cannot see
Sites are planned as averages, and averages hide your worst client. One site with a page builder, a heavy theme and a cache-hostile plugin can eat what four lean sites would have used. Traffic shape is invisible too: ten brochure sites that never spike and ten blogs that catch a link wave are the same input to the form and very different evenings.
When to trust it
Trust the band when the sites resemble each other and their traffic is steady. Raise the spike reserve when the mix includes a shop or anything with a publish-day spike. When one site clearly outweighs the rest, size it separately or give it its own box; per-site averaging is exactly the wrong tool for a fleet with one giant in it.
Where to go next
Run the server size calculator when the question points the other way. The guide to hosting multiple production sites on Cloudways covers the practical side of packing sites onto one box. And if the sites on it are clients, the agency hosting margin calculator tells you what each of those slots actually earns.
FAQ
How is this different from the server-size calculator?
Direction. The server-size calculator starts from sites and answers in gigabytes; this one starts from a box and answers in sites. Both use the same per-site baselines and the same one-core-per-2-GB line, so a box sized for five sites holds five when you run the estimate back the other way.
Why does CPU ever bind before RAM?
Because the planning line gives each core 2 GB of planned RAM. Two cores carry 4 GB of sites however much memory the plan advertises, and at roughly a gigabyte per WordPress site that is three to five sites. Extra RAM on a 2-core box does not raise the CPU line, and the calculator shows both walls so the smaller one is visible.
Why is the answer a band and not a number?
Because sites differ by plugins, theme weight, cache hit rates and traffic, and no constant in the model has measured yours. The band edges floor to whole sites, and the range with its working shown is the honest answer. Raise the spike reserve if the band feels optimistic for your roster.