AS/RS capacity is sized from three hard inputs — SKU count, required order-lines per hour, and the maximum allowable building height — before any vendor shortlist is built, because each axis drives a different machine class and a different price band.
For greenfield distribution centres in the 5,000-40,000 tote range, the dominant 2026 selections split between unit-load stacker-crane aisles at 30-40 m height, mini-load AS/RS at 6-12 m, and pallet shuttle systems for floor-level high-density storage; control software (WCS/WES) typically adds 8-15% to the mechanical budget.
Throughput Sizing: From Order-Lines/Hour to Crane Count
A single mini-load crane sustains 80-160 totes per hour for single-cycle (S/B) operation and 120-250 totes per hour for dual-cycle (S/B/S) operation, where the crane combines a storage and a retrieval move in one trip — the dual-cycle ratio is the single biggest lever on crane count. [S1]
Unit-load stacker cranes handling 1,000 kg pallets at 30 m lift typically rate 25-40 combined cycles per hour, dropping to 18-28 cycles per hour at 40 m lift because the vertical axis dominates the duty cycle. Throughput per crane falls roughly 30% when lift height rises from 12 m to 30 m, which is why high-bay aisle length — not crane speed — is the first number to lock.
Sizing the WCS layer: one crane needs roughly 60-90 minutes of engineering for I/O mapping, and a four-crane mini-load cell typically needs 8-12 weeks of WCS commissioning, including FAT and SAT windows, before the first live order can run end-to-end.
Storage Media, Density and Aisle Geometry
[S2]
Aisle width for stacker cranes is set by the crane base plus 50-80 mm clearance per side; typical mini-load aisles run 1,050-1,400 mm, while unit-load pallet aisles run 1,500-1,800 mm depending on pallet type and load. Storage density for a single-aisle mini-load cell lands in the 0.5-0.8 tote/m² building-footprint range once aisles and service zones are counted, and 1.0-1.5 tote/m² for double-deep unit-load — which is one of the main reasons shuttle-based high-density systems keep eating share in cold-chain and retail DC builds.
Stack height for mini-load is normally capped at 6-12 m of tote column; above 12 m the crane duty cycle degrades faster than the rack saving, so unit-load takes over for full-pallet high-bay. This is also why many greenfield DCs use a mixed strategy — mini-load for slow-movers in totes, with a crossed-roller transfer interface feeding a linear-guide-driven stacker crane aisle for fast-mover pallets.
Selection Criteria Compared: Unit-Load vs Mini-Load vs Shuttle
Unit-load stacker-crane AS/RS suits full-pallet DCs at 25-40 m height, 25-40 cycles/crane/h, capex roughly $50,000-120,000 per crane installed, and serves pharma, automotive and palletised-retail builds where floor-load capacity is high enough for 1,000 kg loads. [S3]
Mini-load AS/RS suits tote-and-carton DCs at 6-12 m height, 80-250 totes/crane/h, capex roughly $15,000-40,000 per crane, and is the default for parts-to-picker, e-grocery back-end, and pharmaceutical picking cells. Shuttle systems suit floor-level 1-4 deep storage at 1,000+ totes per aisle per hour using multiple shuttles in parallel, capex $8,000-25,000 per shuttle, and dominate where throughput per square metre beats height.
For a hands-on comparison of the trade-off between a stacker-crane cell and a multi-shuttle cell on a single-aisle retrofit, the AS/RS sizing logic for pharma write-up walks through the same three axes — throughput, density, height — against a GMP/FIFO constraint set, which is a useful mirror for non-pharma spec-driven sizing.
Control Stack, Interfaces and Integration Risk
WCS/WES scope is the line item most often under-priced at the spec stage: typical integrator offerings split into WCS-only (1-2 PLC brands, simple routing), WCS + WES (multi-crane orchestration, slotting, replenishment), and full WMS-extension stacks. WCS-only for a 4-crane mini-load cell lands in the $80,000-150,000 software + commissioning band; full WCS+WES lands at $250,000-600,000 depending on WMS brand and SKU count. [S4]
Conveyor and sortation-system interfaces are the second integration hotspot — totes arriving on the AS/RS inbound side at 0.6-1.2 m/s need induction lanes with 0.5-1.0 m gaps for crane pickup, and a single 4-crane cell typically needs 3-5 induction lanes plus 2-3 outbound lanes to keep crane wait time under 8% of cycle time.
Risks, Failure Modes and What to Push Back On
Two failure modes dominate greenfield AS/RS handover: under-sized buffer lanes (cranes stall waiting for tote handoff, effective throughput drops 20-30% on day one), and over-specified crane speed (peak-rated cycles/hr fall 15-25% in real duty when acceleration/deceleration zones are accounted for, so rated cycle numbers should be derated before crane-count sizing). [S5]
Standards, Spec Anchors and Sourcing Levers
The governing documents for racking in most US and EU builds are EN 15629 (storage equipment spec), EN 15635 (in-service inspection), and the RMI/ANSI MH16.1 spec for the rack itself; for the crane, EN 15095 covers safety of power-driven racking equipment and IEC 60204-1 covers electrical safety of the machine — pin the integrator to these numbers, not generic "to industry standard". [S1]
On sourcing, the practical levers are aisle-length (each metre adds roughly $400-900 in rack per aisle, $300-600 in crane rail), crane count (one fewer crane at $50,000-120,000 each is the largest cost-out), and tote standard (a 600×400 mm common tote pool reused across receiving, AS/RS and outbound is the most over-looked saving on a multi-DC rollout).
For a fixed-budget envelope of $2-5 million all-in, a 4-crane mini-load cell with WCS + WES typically covers 8,000-15,000 tote locations and 600-1,200 totes/h sustained — anything beyond that band either drops a crane, drops the WES layer, or pushes the project into unit-load territory where the shuttle-system reference above gives a different cost/throughput slope.