The Antminer S21 family offers four distinct SHA-256 machines on the same 5 nm silicon generation: the air-cooled S21 (200 TH/s at 17.5 J/TH), the efficiency-refined S21 Pro (234 TH/s at 15.0 J/TH), the air-cooled flagship S21 XP (270 TH/s at 13.5 J/TH), and the liquid-cooled S21 Hyd (335 TH/s at 16.0 J/TH) — so the right choice depends on your power budget, cooling infrastructure, and hashrate target, not just the headline spec.

Bitmain built the S21 series as the fifth-generation evolution of the Antminer SHA-256 line, and credit where it is due: the jump from the 7 nm BM1398 and BM1366 chips of the S19 era (roughly 29–34 J/TH) to the BM1368 and BM1370 silicon in the S21 family (13.5–17.5 J/TH) is one of the largest single-generation efficiency gains in SHA-256 mining history. D-Central’s team services and repairs all four variants from our Laval bench; every specification on this page is sourced from our dc_miner profile database and bench measurements or is marked as estimated (†). Verify the current Bitmain datasheet before placing a purchase order — specifications can vary between production batches.

For the complete S21 ecosystem — firmware compatibility, error codes, tuning guides, and every variant from the base S21 through the XP+ Hydro — see the Antminer S21 Family Owner’s Hub.

S21 family specification comparison

†Estimated from available specifications; verify current Bitmain datasheet before purchasing. Power figures are wall-draw at nameplate conditions (25 °C ambient). Actual draw shifts with ambient temperature, PSU efficiency, and tuning state. Sources: D-Central dc_miner CPT database (posts 80465, 80466, 80467, 81641); Bitcoin Mining Bible POWER_PROFILES_CATALOG (820 profiles, VNish binary analysis, 2026-03-10); D-Central repair bench records. Chip node per dc_miner S21 Pro profile (BM1370 on 5 nm TSMC; note: some third-party sources classify BM1370 as 3 nm — verify against Bitmain’s current design documents).

Antminer S21 (200 TH/s) — the air-cooled foundation

The base S21 is built on the BM1368, Bitmain’s 5 nm SHA-256 ASIC. Three hashboards (part BHB68603), each carrying 108 BM1368 chips, give the machine 324 ASICs in total. Chips on each board are organized into 12 voltage domains of 9 chips wired in series — so voltage is regulated per domain, not per chip, and a single dead ASIC can take a full chain offline. The control board is an Amlogic A113D (AXG) SoC: no Zynq FPGA, no PIC microcontroller (the “no-PIC” generation). An audio-class DAC sets voltage over I2C in place of the old S19’s PIC-based rail control.

At 200 TH/s and 3,500 W the S21 is a legitimate air-cooled workhorse that needs a dedicated 240 V circuit. Its ~11,940 BTU/h waste heat can be ducted to serve a cold space. The S21 has genuine tuning range: with third-party firmware, performance-mode profiles for BM1368-class hardware run approximately 16.1–16.9 J/TH, and normal-mode operation holds a remarkably flat ~17.9–18.0 J/TH curve across dozens of documented setpoints. Those figures are not fixed presets — the autotuner calculates frequency and voltage at runtime to hit a watt target. Full tuning data for this generation is published in the D-Central ASIC data catalog. For component-level repair see the S21 repair service or the S21 hashboard repair guide.

Antminer S21 Pro (234 TH/s) — the efficiency step-up

The S21 Pro moves to the BM1370 chip — the refined, higher-output successor to the BM1368 — while keeping roughly the same air-cooled chassis and power envelope as the base S21. The result is 234 TH/s at 3,510 W, or 15.0 J/TH: a 14% efficiency improvement over the base S21 for a 3.5 kW draw that is virtually identical. Each of the three hashboards carries 65 BM1370 chips (195 total), fewer than the S21’s 108-per-board count — the chips are larger and more capable, so fewer are needed. The control architecture is the same Amlogic A113D no-PIC design, with I2C DACs handling per-domain voltage. The S21 Pro is a four-fan machine, and fan faults will trigger a shutdown or hard throttle — check all four before troubleshooting a hashrate problem.

The Pro throws off approximately 11,976 BTU/h, and in cold climates (such as the Quebec winters we work in) that exhaust is worth ducting. Like all S21-family air-cooled variants the Pro requires 200–240 V AC — it is not a 120 V machine. For repair, see the S21 Pro repair service. For detailed tuning curves for the BM1370 chip, see the ASIC power profiles database.

Antminer S21 XP (270 TH/s) — the air-cooled flagship

The S21 XP takes the BM1370 further: 270 TH/s at 3,645 W gives 13.5 J/TH — the best efficiency in this four-model comparison and the best efficiency available in any air-cooled SHA-256 machine in the S21 class. The weight is 15.4 kg on the same 400 × 195 × 290 mm footprint as the other air-cooled S21 variants. At 75 dB it is no quieter than the base S21; the XP earns its premium through raw efficiency, not noise or space savings.

A key counterintuitive finding from our spec analysis: the air-cooled S21 XP at 13.5 J/TH is more efficient than the liquid-cooled S21 Hyd at 16.0 J/TH. The Hyd’s liquid cooling enables higher sustained hashrate density and near-silent operation, but efficiency per terahash favours the XP. This matters if your cost structure is dominated by electricity rather than rack space or noise. The S21 XP produces approximately 12,437 BTU/h — useful for heat capture, though at 3,645 W you will want that BTU going somewhere intentional. For repair see the S21 XP repair service.

Antminer S21 Hyd (335 TH/s) — liquid-cooled density

The S21 Hyd (335 TH/s) is the liquid-cooled variant of the base S21 generation, using the same BM1368 ASIC but paired with a closed liquid-cooling loop rather than fans. It runs at a significantly higher sustained power — 5,360 W — because the liquid loop can remove heat faster than air, allowing the chips to run at higher frequencies without thermal throttling. The result is 335 TH/s: 67.5% more hashrate than the base S21, at 53% higher power draw, in a slightly more compact chassis (410 × 170 × 209 mm, 14.2 kg). Without intake fans, acoustic noise is determined by the coolant loop rather than the miner itself — in most properly plumbed installations the S21 Hyd is dramatically quieter than any air-cooled machine in the family.

The infrastructure cost is real: an S21 Hyd requires a coolant distribution unit (CDU) or compatible liquid loop, appropriate hose fittings, and a heat exchanger. This is purpose-built hashcenter equipment. For operators who already have liquid infrastructure, the S21 Hyd delivers high absolute hashrate in a space-efficient package. For those who do not, the capital and operational overhead of building a hydro loop is substantial, and the air-cooled S21 XP will often deliver better economics. For repair and service on liquid-cooled S21 units, contact us through the ASIC repair page to discuss your specific configuration.

Which S21 Should You Buy

Home mining: one machine, standard dedicated space

If you are running a single machine in a garage, basement, or dedicated room on a 240 V circuit, the S21 (200 TH/s) is the most accessible entry point. At 3,500 W it fits on a single 20 A 240 V branch (with appropriate headroom) and its repair ecosystem is the deepest in the family. Efficiency at 17.5 J/TH is the weakest of the four, but the cost-per-machine is lowest and the refurb market for base S21 units is active.

The S21 Pro (234 TH/s) is the step-up for home miners who want meaningfully better efficiency without changing their power infrastructure. At 3,510 W — barely more than the base S21 — it delivers 17% more hashrate at 14% better J/TH. If your electricity cost is the dominant variable, the Pro’s 15.0 J/TH vs the S21’s 17.5 J/TH translates directly into lower cost per sat mined. For most single-machine home setups the S21 Pro is the better buy.

Neither variant is appropriate for 120 V North American household circuits. For true home mining on a standard outlet, an open-source single-board miner in the Bitaxe class is the right tool. The S21 family starts at 3.5 kW — that is a dedicated circuit and a room with real airflow, not a desk appliance.

Hashcenter air-cooled rows: maximize J/TH per PDU port

For operators running air-cooled rows in a hashcenter (what we call a Hashcenter) with standard PDU infrastructure, the S21 XP (270 TH/s) is the standout choice in this family. At 13.5 J/TH it produces 35% more hashrate than the base S21 for only 4% more power draw per unit. Across a row of 10 machines that translates to roughly 700 TH more production on the same circuit breaker compared with the base S21. If your PDU allocation is fixed, the XP is the most efficient way to fill it in the air-cooled S21 class.

The S21 Pro (234 TH/s at 15.0 J/TH) is the right choice where budget constrains the XP premium or where refurb pricing makes the Pro more attractive per watt. Run the numbers against your current electricity rate — at the time of writing, the J/TH gap between Pro and XP is meaningful at any serious hosting rate.

Liquid-cooled or immersion infrastructure retrofit

If your facility already has a liquid cooling loop — or you are building one — the S21 Hyd (335 TH/s) delivers the highest absolute hashrate in the S21 BM1368 generation without the fan noise that creates neighbour complaints or acoustic management costs. The 335 TH/s per unit is 24% more than the air-cooled S21 XP, and the near-silent operation means you can run more units in a given space without noise becoming a facility constraint.

One nuance worth flagging: the S21 Hyd runs at 16.0 J/TH stock, which is less efficient than the air-cooled S21 XP at 13.5 J/TH. Liquid cooling enables density and silence; it does not automatically improve J/TH at the S21 Hyd’s stock spec. The efficiency advantage of liquid cooling for BM1368 silicon shows up more aggressively in the higher-generation S21e Hyd and S21+ Hyd variants, which push into 14–15 J/TH territory. If J/TH is your primary metric and you do not already have liquid infrastructure, the air-cooled S21 XP will typically deliver better economics. If density and noise are the constraints, the S21 Hyd resolves both.

For immersion retrofits (submerged liquid, not closed-loop hydro) there are dedicated S21 Immersion variants optimized for dielectric fluid. The S21 Hyd is a closed-loop liquid machine, not an immersion unit — do not submerge it without manufacturer guidance.

S21 Family Power Profiles

Every machine in the S21 family supports a range of wattage/hashrate operating points — not a single fixed clock frequency. The autotuner in Bitmain’s stock firmware and in third-party firmware images works by binary-searching for the minimum stable voltage per domain and ramping the clock in steps, settling at a target watt point. This means the nameplate figures (200/234/270/335 TH/s) are stock operating points, not ceilings or floors.

For the BM1368-based air-cooled S21, documented third-party firmware profiles span approximately 16.1–16.9 J/TH in performance mode and hold a flat 17.9–18.0 J/TH across normal-mode operating points. For BM1370-based machines (S21 Pro, S21 XP), profiles run to approximately 15.0–15.5 J/TH in normal mode, with further improvements possible through careful per-domain undervolting. For the S21 Hyd, the liquid loop’s thermal stability allows the chip to sustain performance-mode profiles across a wide watt range, with the best BM1368 hydro efficiency documented around 16.1–16.9 J/TH in performance mode and 17.9–18.0 J/TH in normal mode.

D-Central maintains a structured database of 820 documented wattage/hashrate/efficiency profiles across 28 Antminer models, extracted from firmware binaries and verified against bench measurements. This data lives in the open ASIC data catalog at /data/ under CC BY 4.0 — free to use, cite, and build on. Before committing a tuning plan, pull the power-profile curve for your specific model and match the operating point to your electricity rate. Tuning to the wrong point on the curve costs more per sat mined, not less.

All tuning values referenced above are computed at runtime by the firmware autotuner — they are not hardcoded presets. Per-unit silicon quality means two machines of the same model may reach slightly different stable frequencies at the same voltage. See the ASIC power profiles reference for the full efficiency curve data.

Frequently asked questions

What is the difference between the S21 and S21 Pro?

The core difference is the ASIC chip. The base S21 uses the BM1368 (108 chips per board, 324 total), while the S21 Pro uses the refined BM1370 (65 chips per board, 195 total). The BM1370 delivers more hashrate per chip, which is why the Pro reaches 234 TH/s at 15.0 J/TH — a 17% hashrate increase over the S21’s 200 TH/s and a 14% efficiency improvement — with virtually identical wall power draw (~3,500–3,510 W) and the same air-cooled chassis. The control board and voltage-domain architecture are the same in both: Amlogic A113D SoC, no FPGA, no PIC.

Why is the S21 Hyd less efficient than the S21 XP if it uses liquid cooling?

The S21 Hyd (335 TH/s, 16.0 J/TH) uses the older BM1368 chip pushed to 5,360 W. The S21 XP (270 TH/s, 13.5 J/TH) uses the more power-efficient BM1370 chip at 3,645 W. Liquid cooling does not automatically make a chip more efficient per terahash — it allows higher sustained power draw in a stable thermal environment, which enables more absolute hashrate in a given rack space. The S21 Hyd’s value is density (67.5% more hashrate per unit than the base S21) and silence (no fans), not lower J/TH. If electricity cost per TH is the primary metric and your facility supports air cooling, the S21 XP wins on J/TH in this comparison.

Can I run any of these machines on 120 V household power?

No. All four S21 variants require 200–240 V AC. On a standard North American 120 V circuit, the current draw would exceed safe limits even if the PSU’s internal protection didn’t shut it down first. You need a dedicated 240 V branch circuit — typically a NEMA 6-20 or 6-30 outlet wired from a double-pole breaker. For genuine home mining on standard household power, a Bitaxe-class open-source miner running 5–30 W is the appropriate tool.

What is a power profile and can I tune my S21?

A power profile is a wattage/hashrate operating point that the firmware’s autotuner uses as a target. Instead of fixing a clock frequency, modern Antminer firmware (stock and third-party) lets the autotuner find the minimum stable voltage per domain to hit a target watt level — then locks in that point. You can tune an S21-family machine to run at lower wattage for better J/TH (underclock), at higher wattage for more TH/s (overclock), or within the stock efficiency sweet spot. D-Central’s open ASIC data catalog publishes 820 documented watt/hashrate/efficiency points so you can match the operating curve to your electricity rate before you tune, not after. All figures in that dataset are runtime-calculated, not hardcoded presets.

Does D-Central repair all S21 variants?

Yes. D-Central’s in-house ASIC repair bench in Laval has serviced the S21 family since its release. We repair to the component level — diagnosing failed chips, rebuilding domain voltage stages, replacing I2C DACs, and servicing Amlogic control boards — not just board-swapping. Because every S21-family hashboard wires its chips in series, a single failed ASIC can take the whole chain offline; finding and replacing that one chip is exactly where component-level repair pays off versus retiring a board. See the S21 repair service, S21 Pro repair service, and S21 XP repair service for scope and turnaround. For the S21 Hyd or other liquid-cooled variants, contact us through the ASIC repair page.

Where can I see live profitability data for each S21 variant?

The ASIC miner database carries real-time profitability calculations for each model, factoring in current network difficulty, block reward, and a configurable electricity rate. The Bitcoin mining profitability calculator lets you input any wattage/hashrate/electricity combination — useful for modelling tuned operating points rather than just nameplate specs. Before buying any machine, run it against your actual kWh rate at both nameplate efficiency and your target tuned setpoint.