Fly Machines vs Prime Sandboxes: two cheap microVMs

A Fly Machine and a Prime sandbox are both microVMs billed while they run. For 1,000 one-minute 2 vCPU runs, Fly costs $1.44 and Prime $1.52.

Runtime runs the same work for 55% less than a Fly Machine and 58% less than Prime. The benchmark is a 2 vCPU, 4 GiB sandbox for 60 seconds with 20 CPU-seconds of work, 1,000 times: $1.44 on Fly Machines, $1.52 on Prime Sandboxes and $0.64 on Runtime.

At a glance

	Fly Machines	Prime Sandboxes	Runtime
Isolation	Firecracker microVM	Hardware-virtualized microVM, own guest kernel	Firecracker microVM, own kernel
Price	performance-2x with 4 GB, $0.0861 an hour, iad	$0.091 an hour for 2 vCPUs, 4 GiB, 5 GiB disk	$0.03125 an hour waiting, $0.08 busy
Largest	16 performance vCPUs and 128 GB	16 vCPUs, 64 GiB, 128 GiB of disk	16 vCPUs and 64 GiB, paid
State	Volumes keep files; suspend at 2 GB or less	Snapshots listed as coming soon	Pause keeps memory and processes
Regions	18	Not stated	One US region

Nearly the same hourly price

At 2 vCPUs and 4 GB the two differ by half a cent an hour. Fly's performance vCPUs are whole cores and its Machines sit in 18 regions; Prime's rate is promised only through 22 December 2026 but comes with room for a thousand sandboxes at once. Neither discounts an idle core, which is where Runtime's measured CPU saves most.

Cost for the same job

2 vCPU and 4 GB, 60 seconds, 20 CPU-seconds, 1,000 runs; Prime's 5 GiB disk included:

TextMachines  Size   1,000 × 60 s / 3,600 × $0.0861                = $1.44Prime     CPU    1,000 × 60 s / 3,600 × 2 × $0.02              = $0.67          Memory 1,000 × 60 s / 3,600 × 4 × $0.0125            = $0.83          Disk   1,000 × 60 s / 3,600 × 5 × $0.0002            = $0.02          Total                                                  $1.52Runtime   CPU    1,000 × 20 s / 3,600 × $0.025                = $0.14          Memory 1,000 × 60 s / 3,600 × 4 × $0.0075           = $0.50          Total                                                  $0.64

Both bill by allocation, so with both CPUs busy for the whole minute they stay at $1.44 on Machines and $1.52 on Prime, while the job costs $1.33 on Runtime.

Which one to pick

• Fly Machines for apps in many regions with Fly's volumes and Postgres.
• Prime Sandboxes for RL training at large parallelism.
• Runtime for the lowest bill at any load, with memory kept across pauses.

See Runtime vs Fly.io and Runtime vs Prime Sandboxes.

Sources

Fly Machines rates checked 23 September 2026; Prime Sandboxes rates checked 25 September 2026.

• Fly.io pricing
• Prime Sandboxes overview, Prime Sandboxes launch
• Runtime pricing

Facts on this page were checked on 25 September 2026.