gpt-oss-120b on Mac Studio M5 Max 64 GB
116.829B (5.1B active) on 64 GB at 614 GB/s. Hardware details · Model details
CompatibilityDoes not fit
Fits
No
Calculated Requires more than 97% of usable memory. Needs a smaller quantization or more memory.
Recommended quantization
MXFP4
Calculated gguf
Memory required
66.5 GB
Calculated of 54.4 GB usable — 122%
Max practical context
0
Calculated model supports 128K
Memory budget at 8K context
Model weights ⓘ
63.0 GB Calculated
KV cache ⓘ
0.6 GB Calculated
Runtime overhead ⓘ
2.9 GB Estimated
Total required
66.5 GB Calculated
Usable memory ⓘ
54.4 GB Assumption
Headroom
-12.1 GB Calculated
No quantization in our catalogue fits this machine.
Unified memory: weights, KV cache and the OS share the same 64 GB pool. We assume 85% is addressable by the GPU, which matches a headless machine with the wired-memory limit raised.
Mixture of experts: all 116.829B parameters must be resident in memory even though only ~5.1B are active per token. Memory follows total parameters; speed follows active parameters.
PerformanceEstimated0/10
Decode (generation)
—
Estimated calculated, not measured
Prefill (prompt)
—
Estimated
TTFT at 8K
~6.7–13.9 s
Estimated time to first token
Power while generating
118 W
Measured
No throughput figures for this pairing. We do not estimate performance for a model that cannot load: a tokens-per-second number for a configuration that will never run is noise, not data. The alternatives below are machines that can actually run gpt-oss-120b.
How the estimate is calculated
- Decode: reading 5.1B active parameters at 4.25 bits/weight takes 5.04 ms at 614 GB/s x 88% achieved efficiency.
- MoE routing penalty of 15% applied: expert gathers are less bandwidth-efficient than a dense sweep.
- Prefill: 120 TFLOPS (FP16) x 1.69 calibrated against measured prefill on this platform x 22% assumed model-FLOPs utilisation, divided by 2 x 24.4B parameters per token.
- Prefill uses 24.4B effective parameters, not the 5.1B active in decode: a batch of hundreds of tokens routes across most of the expert pool.
- MoE decode depends on how well the engine batches expert gathers; real results vary more than for dense models.
- Prefill throughput is highly engine-dependent. Flash attention, batch size and quantized KV all move this number substantially, and for sparse mixture-of-experts models it is the least reliable figure we produce.
- This is a calculated estimate, not a measurement. It assumes a single request, a short prompt, no speculative decoding and a warm model already resident in memory.
Estimator version estimator_v1. Stored with every estimated row so old estimates can be regenerated when the model improves.
This model at other quantizations on this machine
| Quantization | Weights | Total needed | Utilisation | Fit | Max context |
|---|---|---|---|---|---|
MXFP4recommended | 63.0 GB | 66.5 GB | 122% | Does not fit | 0 |
BF16 | 217.6 GB | 225.7 GB | 415% | Does not fit | 0 |
Cost of running gpt-oss-120b on this machineUnited States (federal) · C corporation · 8h/day
Monthly economic cost
$37
Calculated after tax
Codex/Claude Code
$100/mo
≈ $100/mo · local is $63 less
Net cash at purchase
$3,512
Calculated VAT not reclaimable
Total over 5 years
$2,213
Calculated after tax, after resale
Cost per USD/1M tokens
—
Calculated
The $100 comparison uses the Codex Pro 5x / Claude Max 5x plans. A fixed planning conversion is used for USD. This compares monthly spend only: subscriptions have usage limits, local hardware has different capabilities and constraints, and taxes or regional pricing may change the charged amount. Prices checked 7 September 2026.
Monthly breakdown
| Depreciation 2,459 over 5 years, straight-line to a 1,054 residual | $40.98 |
| Electricity 5.3 kWh/month at 0.140/kWh | $0.74 |
| Cost of capital 4.0%/yr on 2,283 average capital employed | $7.61 |
| Monthly cost before tax | $49.33 |
| Electricity tax shield Running costs are deductible business expenses | −$0.16 |
| First-year expensing §179 (100.0%) | −$12.29 |
| Monthly economic cost after tax | $36.88 |
Three different numbers, deliberately
Cash cost
$3,512
Money that leaves the bank account on day one, net of reclaimable VAT.
Accounting depreciation
$40.98/month
$2,459 written down over 5 years to a $1,054 residual.
After-tax economic cost
$36.88/month
Depreciation plus running costs plus cost of capital, less the tax those deductions save. This is the figure to compare between machines.
Assumptions and sources (verified 2026-09-07)
VAT rate
0% Official spec
VAT recoverable
0% Official spec
Federal C-corporation estimate at the flat 21% rate. Pass-through entities should use the sole-proprietor estimate as a rougher proxy.
Effective deduction rate
21.00% Calculated
Headline marginal rate 21.00%.
Depreciation
5 years, straight-line Assumption
Residual value
$1,054 (30%) Assumption
Extrapolated from the three-year curve.
Electricity
$0.140/kWh Assumption
Average power draw
30 W Calculated
Load 118 W for 20% of powered hours, idle 8 W for the rest — a machine that is on is not generating tokens the whole time.
Investment allowances
§179 100.0% Official spec
Worth $738 in total — first-year expensing that replaces later tax depreciation.
Cost of capital
$7.61/month Assumption
- VAT rate: No US federal VAT · verified 2026-09-07
- Marginal tax rate: IRS Publication 542 — Corporations · verified 2026-09-07
- VAT recoverable fraction: site assumption · verified 2026-09-07
- Useful life: IRS Publication 946 — How To Depreciate Property · verified 2026-09-07
- Electricity price: Site assumption · verified 2026-09-07
Federal planning estimate only, not tax advice. State and local income tax, sales/use tax and incentives are excluded. It assumes 100% business use, a Section 179 election, enough business income to use it, and that the full annual limit remains available.
gpt-oss-120b on other hardware
| Hardware | Decode | Price | Fit |
|---|---|---|---|
| Quad RTX 5090 workstation (4x 32 GB) | ~339–487 t/s | €13.999 | Comfortable |
| RTX PRO 6000 Blackwell workstation (96 GB) | ~257–369 t/s | €11.499 | Comfortable |
| RTX PRO 6000 Max-Q workstation (96 GB, 300 W) | ~257–369 t/s | €11.999 | Comfortable |
| Mac Studio M5 Ultra 512 GB | ~203–293 t/s | €12.599 | Comfortable |
| Mac Studio M5 Ultra 256 GB | ~203–293 t/s | €9.599 | Comfortable |
| Mac Studio M5 Ultra 96 GB | ~203–293 t/s | €6.599 | Fits |
Nearest alternatives to the Mac Studio M5 Max 64 GB
Page quality score (why this page is or is not indexed)
6/13 — marked noindex. Generated pages are gated so we do not ask a search engine to rank a page with nothing computed to say. The directive is emitted in the page head via the metadata API, not in the body, so it is authoritative.
- Has a memory-fit calculation (+3)
- Has a price (+2)
- Has an ownership economics calculation (+2)
- Well connected (16 internal links) (+1)
- No benchmark or estimate
- Model does not fit and there is no measurement to explain why it is worth knowing (-2)
- Fails a hard requirement: an indexable page needs both a compatibility calculation and at least one benchmark or estimate.