Antminer S21 XP vs WhatsMiner M66S: Flagship ASIC Comparison

The Antminer S21 XP (270 TH/s, 13.5 J/TH, air-cooled) and the WhatsMiner M66S (298 TH/s, 18.5 J/TH, immersion-only) are the current SHA-256 flagships from Bitmain and MicroBT respectively — but they serve fundamentally different facility types. The S21 XP demands no special cooling infrastructure and delivers the best efficiency in its class; the M66S pushes higher absolute hashrate per unit but requires a full immersion or hydro cooling system before the first block is ever submitted. Choose based on your facility first, your efficiency target second.

Both manufacturers have earned their reputations across generations of SHA-256 hardware. Bitmain pioneered mass-market ASIC production with the S9 series and has refined the air-cooled form factor through S17, S19, S19 XP, and now the S21 generation. MicroBT built their standing on competitive performance, an integrated-PSU approach in earlier air models, and — with the M6x series — a deliberate move toward liquid-cooled high-density Hashcenters. Every specification on this page is sourced from manufacturer datasheets (as of the dates noted) or D-Central’s service database; hashrate varies ±3% and wall power varies ±5% in practice — verify the current datasheet before purchasing. No prices are quoted here; pricing is volatile and must be sourced directly.

D-Central’s Montreal workshop repairs both platforms. The observations on repair complexity below are based on bench experience, not editorial preference.

Side-by-side specification table

† Hashrate varies ±3%; wall power varies ±5% per manufacturer tolerance. Verify the current Bitmain and MicroBT datasheets before any purchase decision.

The cooling divide: why this comparison is not apples-to-apples

The single most important fact in this comparison is the cooling method. The Antminer S21 XP ships with integrated fans and runs in any standard mining environment — a home setup, a co-location rack, or a purpose-built Hashcenter. No coolant, no tank, no pump infrastructure required.

The WhatsMiner M66S is immersion-only. Deploying an M66S requires a dielectric fluid tank, circulation pumps, a heat exchanger or chiller, fluid management systems, and the facility space to house all of it. That infrastructure is a substantial capital and operational commitment on top of the machine cost itself.

This means the two units do not compete for the same buyer:

• Air-cooled Hashcenters and home miners: the M66S is simply not an option without immersion build-out. The S21 XP is the relevant air-cooled flagship comparison.
• Immersion Hashcenters: both the S21 XP Immersion variant (473 TH/s at approximately 12 J/TH) and the M66S are viable choices. The S21 XP Hyd reaches significantly higher efficiency in that environment.

For an air-cooled apples-to-apples comparison with MicroBT hardware, the WhatsMiner M60S+ (up to 210 TH/s air-cooled) is the more direct peer. This page compares the S21 XP and M66S as the flagship products each manufacturer offers at their respective performance ceiling — understanding that they presuppose different facility types.

Efficiency: the S21 XP’s defining advantage

At 13.5 J/TH, the Antminer S21 XP represents the most efficient air-cooled Bitcoin miner Bitmain has shipped to date and one of the most efficient SHA-256 machines in the industry regardless of cooling type as of its release. The BM1370 chip (TSMC 5 nm, fifth-generation Antminer architecture) delivers roughly 220+ chips per hashboard across four boards — a configuration that reflects years of incremental refinement from BM1387 through BM1366 to BM1368 and now BM1370.

The M66S at 18.5 J/TH is notably less efficient than the S21 XP even though the M66S uses immersion cooling, which typically enables lower operating temperatures and tighter voltage tolerance. The higher power draw (5,513 W vs 3,645 W) and lower efficiency ratio reflect a different design priority: the M66S is engineered to push high absolute hashrate in an immersion-capable Hashcenter, accepting a higher energy cost per terahash in exchange for more hash per physical unit.

The M66S+ (318 TH/s at 17 J/TH) and M66S++ (348 TH/s at 15.5 J/TH) narrow the gap somewhat, but the S21 XP at 13.5 J/TH remains more efficient per terahash than any M66S variant. For operators whose electricity cost is their primary variable, efficiency wins.

Hashboard architecture

Antminer S21 XP: BM1370 on TSMC 5 nm

The BM1370 is Bitmain’s fifth-generation SHA-256 ASIC, fabricated on TSMC’s 5 nm process node. The S21 XP carries three hashboards, each populated with 91 BM1370 chips (273 total) in a configuration that eliminates the PIC microcontroller used in older Antminer families. Voltage control passes through a TAS5782M audio DAC (the “No-PIC” architecture introduced with the S21 base), managed by an Amlogic AXG (am3-aml) control board running Linux-based firmware.

A useful reference point: earlier S21-generation chips ran at 17.5 J/TH (S21, BM1368) and 15.0 J/TH (S21 Pro, BM1370 lower-bin). The S21 XP extracts 13.5 J/TH from the same BM1370 silicon through tighter binning and higher operating frequencies — a direct result of Bitmain’s silicon binning process, which has improved steadily since the S9 era. For the full S21 family context including the S21+ and Hyd variants, see our Antminer S21 family comparison.

WhatsMiner M66S: K-series ASIC, proprietary architecture

MicroBT’s M60-generation hardware uses the K-series ASIC family (part numbers in the K32A/K55A/K88A range on the M60S). The process node for the M66S-series chips has not been publicly disclosed by MicroBT as of the time of writing — treat any third-party claims about nm node with caution unless sourced directly from a MicroBT datasheet. The control board runs an Allwinner H616 SoC with OpenWrt-based firmware, and hashboard communication uses an SPI/UART protocol rather than Antminer’s I²C-based chain architecture.

MicroBT has invested heavily in immersion-optimized design with the M66S family, including heat spreader geometry and coolant flow paths optimized for dielectric fluid — which explains why these units are not field-convertible to air cooling.

Firmware and software ecosystem

The Antminer S21 XP benefits from a broader third-party firmware ecosystem. VNish firmware supports the S21 XP family, adding power-targeted autotuning (the firmware calculates optimal frequency and voltage per chip at runtime — these are not fixed presets). Third-party options expand the tuning range, enable per-domain monitoring, and add remote management capabilities that stock Bitmain firmware does not expose. See the firmware comparison page for a full feature matrix across BraiinsOS+, VNish, LuxOS, and stock firmware.

The WhatsMiner M66S runs MicroBT’s proprietary firmware exclusively. As of the current state of the community’s reverse-engineering work, no confirmed third-party firmware exists for the M66S series. MicroBT’s firmware container uses a chained-AES encryption scheme that has not been publicly broken. The official MicroBT firmware does include a web-based tuning interface and supports basic pool configuration and monitoring; for WhatsMiner-specific firmware guidance see our WhatsMiner firmware guide. The closed nature of WhatsMiner firmware is a meaningful consideration for operators who rely on power-targeting or advanced per-board diagnostics.

Repair and parts ecosystem

The Antminer platform has a substantially larger global repair and parts ecosystem than WhatsMiner. BM1370 chips are available from multiple aftermarket suppliers. Hashboard schematics circulate in the repair community, and independent repair shops on multiple continents stock S21-generation components. D-Central’s Montreal workshop services all S21 variants including the XP, with access to BM1370 replacement chips and hashboard-level repair capability. Visit our ASIC repair services page for current service scope.

WhatsMiner repair is more concentrated. Parts availability for M66-generation boards is narrower, with fewer independent suppliers globally. The proprietary K-series ASICs are not as widely distributed in the aftermarket as BM1370 variants. Importantly, M66S immersion units are not designed for field repair in the way air-cooled units are — servicing requires extracting the unit from the fluid bath, which adds procedure complexity. MicroBT does have authorized service partners, but the global network is smaller than Bitmain’s.

Neither of these is a disqualifying factor for a well-resourced industrial operator, but for smaller operations or those in locations with limited access to authorized service, the S21 XP’s repair ecosystem depth is a real operational advantage.

Power infrastructure requirements

Power draw is a decision input before any efficiency calculation. The S21 XP at 3,645 W can run on a standard 240 V / 20 A circuit (in North America) with headroom to spare. The M66S at 5,513 W requires a dedicated high-amperage circuit and appropriate breaker sizing — and that 5,513 W does not account for the circulation pumps, chiller compressor, and cooling system fans that an immersion setup requires on top of the machine itself. A complete single-unit immersion deployment can easily add 500–800 W of auxiliary load depending on the tank design and ambient temperature.

For pool configuration and protocol setup common to both machines, see our mining pools guide and the Stratum V1 vs Stratum V2 comparison.

Honest verdict: match the machine to the facility

There is no universally superior choice between the S21 XP and the M66S. The right answer is determined by infrastructure, not spec sheets.

• Air-cooled facility, efficiency-first: The Antminer S21 XP is the stronger choice. At 13.5 J/TH it is more efficient than any M66S variant, it slots into existing infrastructure without modification, its repair ecosystem is deeper, and it carries broader firmware flexibility. Bitmain has built and refined this form factor across five chip generations, and the S21 XP is the most mature expression of that lineage.
• Immersion Hashcenter, hashrate-density priority: The M66S family is purpose-designed for this environment, and MicroBT has engineered their M66-series hardware around immersion thermal paths. Operators already running immersion tanks who want to maximize hash per unit footprint should evaluate the M66S++ at 348 TH/s and 15.5 J/TH, then compare it against the Antminer S21 XP Hyd (473 TH/s, ~12.0 J/TH) — which would be the more natural Bitmain competitor for an immersion deployment.
• Mixed or uncertain environment: The S21 XP’s flexibility wins. It works today with existing infrastructure, can be redeployed easily, and does not obligate the operator to maintain a cooling system that costs more to service than the machines themselves if the facility changes.

Both Bitmain and MicroBT continue to iterate rapidly. Treat any current specification as valid for the production batch in question, and verify against the latest datasheet before committing to a large order.

Frequently asked questions

What is the efficiency difference between the S21 XP and the M66S?

The Antminer S21 XP operates at approximately 13.5 J/TH under stock Bitmain firmware settings (270 TH/s at 3,645 W). The WhatsMiner M66S operates at approximately 18.5 J/TH (298 TH/s at 5,513 W). The S21 XP is meaningfully more efficient per terahash, though the M66S is an immersion-cooled unit operating in a different facility category. The M66S++ narrows the gap to ~15.5 J/TH at 348 TH/s. All specifications per manufacturer datasheets; verify current sheets before purchasing.

Can you run third-party firmware on the WhatsMiner M66S?

As of the time of writing, no confirmed third-party firmware exists for the M66S series. MicroBT uses a proprietary chained-AES firmware container on their M6x hardware. The Antminer S21 XP, by contrast, has confirmed support from VNish and other third-party options. See the firmware comparison for current support status.

Is the WhatsMiner M66S air-cooled?

No. The M66S is designed exclusively for immersion or hydro cooling and cannot be operated with standard air cooling. The air-cooled WhatsMiner flagship in the same generation is the M60S+ series (up to approximately 210 TH/s). For WhatsMiner air-cooled setup and firmware guidance, see our WhatsMiner firmware guide.

What chip does the WhatsMiner M66S use?

The M66S uses a K-series ASIC from MicroBT (chip part number series K32A/K55A/K88A in the M60 generation). MicroBT has not publicly disclosed the process node (nm) for the M66S generation chips. The Antminer S21 XP uses the BM1370, fabricated on TSMC’s 5 nm process.

How many hashboards does each machine have?

The Antminer S21 XP has three hashboards, each populated with 91 BM1370 chips (273 total). The WhatsMiner M60-family architecture typically uses three hashboard slots (per the M60S configuration); verify the M66S-specific datasheet for confirmation as immersion variants may differ.

Can the Antminer S21 XP run in immersion cooling?

Bitmain offers a dedicated immersion variant — the Antminer S21 XP Immersion — designed specifically for dielectric fluid immersion at approximately 473 TH/s and ~12.0 J/TH. The standard S21 XP air unit is not field-convertible to immersion without hardware modification and is not recommended for immersion deployment without Bitmain’s immersion-specific hardware variant.

What is the repair ecosystem like for each machine?

The Antminer S21 XP benefits from a broader global repair and parts ecosystem: BM1370 replacement chips, wider independent repair shop coverage, and more circulating documentation. WhatsMiner M66S parts are available but more concentrated — the global authorized service network is smaller and independent repair is more complex given the proprietary chip architecture and immersion-specific board design. D-Central offers repair services for both platforms from our Montreal workshop.

Which miner is better for a home setup?

The Antminer S21 XP is the relevant choice for any non-immersion home setup. At 3,645 W it runs on standard high-amperage wiring. The M66S cannot be home-deployed without immersion infrastructure. For home miners evaluating overall options, the best Bitcoin miners guide covers the full range of configurations including lower-wattage units suited to residential settings.