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OpenVPN’s current sizing guidance estimates about 12 MHz of CPU per 1 Mbps of traffic on a modern processor with AES-NI. Without AES-NI, plan for roughly four times as much CPU. Memory is easier to estimate: begin at 1 GB and add approximately 1 GB for every 150 connected devices, while treating that figure as a starting point rather than a capacity guarantee.
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The four measurements that determine an OpenVPN server’s size
“How many users?” is not enough information to size a VPN server. A device that is connected but idle consumes far fewer resources than a device continuously transferring files. Before choosing a virtual machine or physical server, establish these four measurements:
- Sustained throughput: the Mbps that must cross the VPN for minutes or hours, not just a short burst.
- Concurrent devices: laptops, phones, routers and other clients that maintain sessions at the same time.
- Routing mode: full-tunnel internet access sends all client traffic through the server; split tunneling sends only private-network traffic through it.
- Encryption and packet-processing load: cipher operations, packet rate, NAT, retransmissions, logging and management services all affect CPU and memory use.
A server can have enough RAM for hundreds of idle connections yet fail when a smaller group starts video calls or large downloads. Conversely, a high-CPU machine cannot overcome a provider’s bandwidth cap or a VPN subnet that has run out of addresses.
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CPU requirements and the 12-MHz-per-Mbps estimate
Encryption and decryption are normally the largest CPU costs. OpenVPN’s Access Server guidance uses a rough planning figure of 12 MHz for each 1 Mbps of traffic on a modern CPU with AES-NI hardware acceleration. This is an estimate under stated assumptions, not a guaranteed benchmark for every instance type, cipher, packet size or workload.
How to apply the formula
Multiply your expected sustained throughput by 12 MHz:
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Required CPU frequency ≈ sustained Mbps × 12 MHz
For example, 100 Mbps of sustained encrypted traffic corresponds to approximately 1,200 MHz (1.2 GHz) of calculated CPU capacity. The estimate is about total processing capacity, so a 2-vCPU instance may behave differently from a single-core processor with the same nominal frequency.
OpenVPN illustrates the arithmetic with a 4-core, 3 GHz processor: 4 × 3,000 MHz equals 12,000 MHz, which under its assumptions could process approximately 1,000 Mbps. That example is a sizing illustration, not a promise that every 4-core 3 GHz server will deliver 1 Gbps in production.
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When AES-NI is missing
If the processor does not support AES-NI, OpenVPN says encryption is slower and recommends roughly quadrupling the CPU calculation. A 100 Mbps target would therefore be planned at about 4,800 MHz instead of 1,200 MHz. Check that AES-NI is exposed to your virtual machine; a capable physical host does not help if the hypervisor masks the instruction set.
Other CPU variables
- Packet rate: many small packets can consume more CPU than the same Mbps in large packets.
- Routing and NAT: full-tunnel internet service generally requires more packet processing than a tunnel limited to a few private subnets.
- Packet loss: retransmissions increase work and can make an otherwise adequate CPU appear undersized.
- Encryption mode and software versions: cipher choices, kernel behavior and OpenVPN configuration change results.
- Competing processes: backups, web services, monitoring agents and databases contend for CPU time.
Use the 12-MHz rule to choose an initial instance, then verify CPU utilization and throughput under your actual traffic pattern.
RAM requirements: a practical starting model
OpenVPN’s Access Server guidance says to start with at least 1 GB of RAM and add about 1 GB for every 150 connected devices. This makes memory relatively predictable compared with CPU, but it is still an estimate. NAT-heavy traffic, extensive logging, management services, databases and large connection counts can raise consumption.
| Deployment situation | Starting memory guidance | Important qualification |
|---|---|---|
| Simple deployment for about 10 users | About 1 GB RAM | OpenVPN’s cloud example for a light workload; not a universal minimum |
| Up to roughly 150 connected devices | At least 1 GB RAM, with monitoring | Actual use varies with NAT, logging and services |
| About 300 connected devices | Roughly 2 GB as a planning estimate | Derived from the 1 GB per 150-device rule; validate under load |
| Large, clustered or IoT deployment | Size from measured workload and scenario testing | Connection count and packet volume can dominate the simple estimate |
Do not confuse “connected devices” with “active users.” A phone that keeps a tunnel alive while idle still occupies a connection, while an active backup client can consume substantial CPU and bandwidth without creating many additional sessions.
Bandwidth: size the link for traffic, not account count
Your bandwidth requirement depends on how much data you plan to route through the VPN. Estimate the sustained ingress and egress load, then add headroom for bursts. Full-tunnel browsing, cloud backups, large file transfers, video meetings and camera feeds can create prolonged demand. Split-tunnel access to a few internal applications usually requires much less internet capacity.
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A simple capacity calculation
If 100 users share a 1 Gbps server link and all saturate it simultaneously, each would receive an average of 10 Mbps:
1,000 Mbps ÷ 100 users = 10 Mbps per user
Real usage is often lower because only a subset of users transfers data at one time. Do not rely on that assumption for workloads with synchronized backups, software rollouts or scheduled media uploads; those events can make many users active together.
What to confirm with a cloud provider
- The guaranteed or baseline external bandwidth, not only the instance’s advertised port speed.
- Burst limits and how long bursts may continue.
- Traffic throttling, monthly transfer quotas and overage policies.
- Whether public IPv4 addresses, load balancers or egress services add separate limits or charges.
- Regional network behavior and the path between the server and your users or private networks.
A 1 Gbps virtual network interface does not guarantee 1 Gbps of encrypted VPN throughput. The CPU, packet rate, provider policy and destination networks all remain part of the bottleneck.
How many users can one OpenVPN server handle?
There is no single user number because “connected” and “actively transferring” are different loads. OpenVPN’s infrastructure guidance recommends keeping client-internet traffic to fewer than 1,000 VPN connections per Access Server installation as a practical NAT limit. The default maximum is 2,048 connections; it can be increased for minimal-traffic use cases, but doing so carries a performance cost.
VPN subnet capacity can become the first limit
A 24-bit VPN subnet provides up to 254 simultaneous client addresses. That address pool can be exhausted before CPU or RAM reaches a critical level. If you need more than 254 simultaneously addressed clients, plan a larger or multiple subnets and confirm that your routing, firewall rules and client configuration support the design.
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When to use multiple servers
Use multiple Access Server nodes when throughput, availability or connection count exceeds what one node can safely provide. Distribute users across nodes, provide independent failure domains and avoid assuming that simply raising the connection ceiling will solve NAT, subnet or bandwidth constraints. High-throughput designs should be tested as a cluster or load-distributed service rather than treated as one oversized host.
Cloud instance guidance
OpenVPN’s cloud examples describe 1 GB RAM and 1–2 vCPUs for about 10 users in a simple deployment. They describe T-series instances as generally acceptable for demands up to roughly 200 Mbps, while C-series instances are intended for higher sustained bandwidth. Provider throttling, burst-credit behavior and external-network conditions can change the result, so these categories are starting points rather than guarantees.
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OpenVPN also reports that external traffic above about 1 Gbps may encounter protective measures in AWS environments and recommends multiple Access Servers for more than 1 Gbps. That observation is specific to the provider environment and scenario described; it should not be generalized to every cloud or region.
Cloud prerequisites to verify
- A stable public IP address or an equivalent reachable endpoint.
- Permission to create and use a TUN device and the required kernel/network capabilities.
- Security-group, firewall and routing rules that allow VPN control and data traffic.
- Provider policies on IP forwarding, packet filtering and source/destination checks.
- Bandwidth ceilings, burst behavior and egress pricing for the selected instance and region.
Keep Access Server off a busy shared-purpose host. OpenVPN advises against co-locating it with firewall or configuration-management applications that may interfere with packet filtering or consume resources unpredictably.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Performance features that change the sizing decision
AES-NI hardware acceleration
AES-NI is a major CPU selection criterion. With it available and used by the operating system and OpenVPN, AES encryption requires substantially less processor time than software-only encryption. Confirm support inside the guest rather than relying on the host’s advertised processor model.
Data Channel Offload (DCO)
Data Channel Offload can improve performance by moving data-channel cryptography into kernel space when the feature is supported and enabled on both the server and clients. DCO is not an automatic upgrade for every installation: compatibility, operating-system support and client versions determine whether it can be used. Treat it as an optimization after confirming functional compatibility, not as a substitute for bandwidth and CPU planning.
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Full tunnel versus split tunnel
Full-tunnel mode routes a client’s internet traffic through the VPN, increasing throughput, NAT and egress requirements. Split-tunnel mode sends only selected private-resource traffic through the server, often reducing bandwidth demand and allowing a smaller instance. Document which networks and applications use the tunnel before calculating capacity.
Worked sizing examples
Light remote-access group
For approximately 10 users with occasional private-network access, begin with 1 GB RAM and 1–2 vCPUs, as in OpenVPN’s simple cloud example. If traffic remains well below 200 Mbps and users are not running synchronized transfers, a general-purpose burstable instance may be suitable. Monitor CPU, memory, tunnel latency and provider bandwidth rather than assuming the smallest instance will remain adequate.
100 Mbps sustained full-tunnel target
The 12-MHz estimate produces about 1,200 MHz of CPU capacity with AES-NI. Select at least 1 GB RAM plus headroom for the actual device count, and choose a network service whose sustained external bandwidth comfortably exceeds 100 Mbps. Without AES-NI, plan for roughly 4,800 MHz of CPU capacity.
200 Mbps deployment with many idle devices
At 200 Mbps, the AES-NI estimate is about 2,400 MHz. A T-series instance may fit OpenVPN’s “up to roughly 200 Mbps” guidance, but burst credits and provider throttling must be checked. If hundreds of devices remain connected, apply the memory-per-device estimate and verify that the VPN subnet has enough addresses.
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A single-node design becomes risky at this point. OpenVPN’s AWS guidance notes protective measures above about 1 Gbps and recommends multiple Access Servers. Use multiple nodes, distribute clients, validate the provider’s egress policy and test failover before placing production traffic on the design.
A repeatable sizing procedure
- Count devices: include laptops, phones, routers and always-on appliances that may maintain sessions.
- Classify routing: record which users require full-tunnel internet access and which need only private resources.
- Estimate sustained Mbps: include simultaneous video, file transfers, backups and camera traffic; add burst headroom.
- Calculate CPU: multiply sustained Mbps by 12 MHz when AES-NI is available; multiply that result by about four if it is not.
- Calculate RAM: start at 1 GB and add about 1 GB per 150 connected devices, then reserve memory for logging, databases and other services.
- Check limits: compare the result with the provider’s bandwidth, burst, NAT and public-IP policies, and with the VPN subnet’s address capacity.
- Choose architecture: use one node for modest loads; use multiple nodes for high throughput, high availability or connection counts approaching practical limits.
- Load-test and observe: measure CPU saturation, memory pressure, packet loss, latency, throughput and tunnel stability while reproducing the busiest expected workload.
Symptoms of an undersized server and corrective actions
| Symptom | Likely constraint | What to check or change |
|---|---|---|
| Throughput plateaus while CPU is near 100% | Insufficient encryption or packet-processing capacity | Confirm AES-NI, reduce competing workloads, move to faster vCPUs or add nodes |
| Users report slow internet but VPN CPU is low | Provider egress cap, burst exhaustion or destination congestion | Review instance bandwidth policy and external network paths |
| New clients cannot connect despite available CPU and RAM | Subnet addresses, connection ceiling or firewall rules | Check the 254-address /24 pool, connection limits and routing configuration |
| Memory steadily rises during busy periods | NAT, logging, database or management overhead | Inspect service usage, reduce unnecessary logging and add RAM or separate services |
| Performance varies sharply by time of day | Burst-credit depletion or shared-host contention | Review provider metrics and move to a predictable-performance instance |
What to monitor after deployment
Track CPU utilization by core, AES-NI availability, RAM and swap, encrypted ingress and egress, packet loss, retransmissions, connection count, NAT table usage and VPN subnet allocation. Alert before the server reaches its limits: sustained CPU saturation, swap activity, repeated packet loss or a connection count approaching the practical ceiling are capacity signals, not merely troubleshooting details.
Recalculate when you add full-tunnel users, introduce backup or video workloads, change cipher or DCO settings, move regions or discover a provider bandwidth policy. Capacity planning is complete only when the measured production workload fits inside the server’s CPU, memory, address, NAT and network limits with room for its busiest expected period.
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