Zone Picking vs Pick-and-Pass: Where Each Wins on Throughput

The new pick-and-pass line ran beautifully in the vendor's simulation and jammed on its third Monday. Totes queued forty deep in front of zone A, the pickers in zones C and D stood idle waiting for work, and the 17:00 parcel cutoff left with 600 orders still on the conveyor. Nothing had broken. The order mix had shifted toward the fast movers slotted in zone A, and one zone now carried 40% of the work with 25% of the people.

Choosing between pick-and-pass and parallel zone picking is a calculation, not a preference. Both divide the pick area into zones; they differ in what moves between zones and where the order is put back together. With an order profile, a pick-time standard and a labour rate, a site team can predict which method gives the lower cost per order and which one will meet the cutoff, before buying a metre of conveyor.

• What this guide covers: the two zone methods, the order-profile tests that separate them, zone workload arithmetic, tote travel, rebalancing and consolidation.
• What it adds: a worked four-zone module costed in dollars and rupees, a scanner-fault incident, a formula block, and ownership and commissioning tables.

Two ways to split an order across zones, and what travels between them

In pick-and-pass, one container carries the order from zone to zone until every line is picked. In parallel zone picking, every zone picks its share of the order at the same time into its own container, and the pieces meet later at a consolidation point.

The academic literature uses the same split. De Koster, Le-Duc and Roodbergen's 2007 review of order picking describes progressive zoning, where a picker finishes their part and hands the tote to the next zone, and notes this is also called pick-and-pass. It calls the alternative parallel or synchronised picking, where several pickers start the same order in their own zones and the partial orders are merged after picking.

Pick-and-pass, which de Koster, Le-Duc and Roodbergen's 2007 order-picking review calls progressive zoning, completes each order in one tote that visits every required zone in sequence; parallel zone picking splits the order across zones simultaneously, so a four-zone order creates up to four totes that must be merged before packing.

Everything else follows from that difference. Pick-and-pass avoids a merge step but makes the order wait for each zone in turn, so cycle time is the sum of the zone visits plus conveyor transit. Parallel picking finishes when the slowest zone finishes, but pays for a consolidation step and for tracking several containers per order. The worked numbers later in this article put a price on both.

• Pick-and-pass suits: small orders that fit one tote and touch two or three zones, on a conveyor with zone skipping.
• Parallel zone picking suits: large orders spanning many zones, temperature-separated areas, and tight cutoffs where waiting in sequence is too slow.
• Neither suits: single-line orders in very high volume, which usually go faster as batches sorted downstream.

Order profile tests that decide which method fits a site

Five measures taken from three months of order history decide the method for most sites. Pull them before talking to equipment suppliers, because each supplier's preferred answer tends to match what it sells.

Our article on batch versus wave picking covers a different decision: how orders are grouped and released to the floor. That choice sits above this one. A site can release orders in waves or continuously and still route each released order through zones by either pick-and-pass or parallel picking, so settle the release logic first and then use the tests below.

Zone workload arithmetic and the bottleneck zone

A zone line can only run as fast as its busiest zone. Calculate the workload of each zone in picker-hours, compare it with the hours staffed there, and the zone with the highest ratio sets the capacity of the whole pick-and-pass line.

Each zone's workload has two parts. Line work is the number of lines picked in the zone times the time per line, including walking within the zone. Visit work is the number of order containers entering the zone times the fixed time to receive, scan, and push on each one. Visit work is what makes many small zones inefficient, because every zone an order touches adds a fixed handling cost.

# Zone workload, staffing and line capacity for one shift
# z           = zone index
# L_z         = order lines picked in zone z per shift
# V_z         = order containers visiting zone z per shift
# t_line      = seconds per line including in-zone walking (engineered standard)
# t_visit     = seconds per container visit: receive, scan, confirm, push
# H           = productive hours per picker per shift
# P_z         = pickers assigned to zone z
# N           = orders released per shift

W_z      = (L_z * t_line + V_z * t_visit) / 3600          # picker-hours of work in zone z
util_z   = W_z / (P_z * H)                                  # above 1.0 means zone z cannot keep up
cap_line = N / max(util_z)                                  # orders per shift the line can finish
balance  = max(W_z / P_z) / mean(W_z / P_z)                 # 1.00 is perfect; above 1.15 needs action
WIP      = lambda * W_sys                                   # Little's law: totes in the loop
           # lambda = orders per minute, W_sys = minutes from induction to pack

The utilisation figure tells you where to act. A zone above 1.0 will build a queue that grows all shift; a zone below 0.7 has pickers who will be idle for long stretches. Parallel zone picking uses the same workload formula, but a busy zone delays only its own share of each order, while the other zones keep picking ahead.

Pick-and-pass line capacity equals orders released divided by the highest zone utilisation, where utilisation is zone picker-hours of line and visit work over staffed hours; a balance ratio above 1.15 between the busiest and average zone means rebalancing slotting or staffing before adding conveyor.

Worked throughput for a four-zone module in dollars and rupees

The worked example uses one order profile and two labour markets, with illustrative rates. It shows that the same zone design can be under-capacity at even staffing and comfortable at balanced staffing, and it prices both methods per order.

The module has four zones. The shift releases 3,600 orders averaging 4.2 lines, so 15,120 lines. Zone A holds the fast movers and takes 40% of lines; B takes 25%, C 20% and D 15%. Orders visit A 2,700 times, B 1,980, C 1,620 and D 1,260, so the average order touches 2.1 zones. The standard is 18 seconds per line and 12 seconds per container visit, with 7 productive hours per picker.

For this profile, pick-and-pass wins on labour by about 18%, provided zone A is staffed for its real workload. The result reverses when orders touch more zones: at an average of 3.5 zones per order, visit work rises sharply in pick-and-pass and cycle time grows with every handoff, while parallel picking's consolidation cost grows only with the number of totes. Rerun the table with your own profile; the order mix, not the method, decides.

Labour cost is not the only line. Pick-and-pass usually needs a powered conveyor loop with scanners and diverts at each zone, while parallel picking needs a consolidation area, such as a put wall or merge lanes. Capital per order differs by site and supplier, so get both quoted against the same order profile.

Tote travel, zone skipping and conveyor accumulation

In pick-and-pass, the conveyor is the work queue. Every tote on it is an order in progress, so the conveyor must hold enough totes to keep each zone busy and must skip zones an order does not need, or travel time swamps the saving from avoiding consolidation.

Zone skipping works by a scan at each zone entry. The warehouse execution software reads the tote label, checks whether the order has lines in that zone, and either diverts the tote onto the zone's pick spur or sends it straight past. Without skipping, every tote stops at every zone, and a 2.1-zone average order handles like a four-zone order.

• Accumulation length: size spurs from Little's law; 3,600 orders over 7 hours is 514 an hour, and a 9-minute average from induction to pack puts about 77 totes on the line, with the largest share queued in front of the busiest zone.
• Tote fill limit: cap cube at about 85% of the tote so later zones can still add items, and route predicted overflow orders to a two-tote path at release.
• Tote weight limit: set a maximum weight that pickers can push and lift repeatedly; the recommended weight limit method published by NIOSH starts from a 23 kg load constant and reduces it for reach, height, frequency and grip.
• Recirculation loop: give totes that miss a divert a path back to the start rather than a dead end, and count recirculations by zone as a scanner health measure.

Rebalancing zones when the weekly order mix shifts

Zone balance decays as the order mix changes, so build rebalancing into the weekly routine rather than waiting for a queue. Three levers exist, in increasing order of effort: move people, move zone boundaries, or move SKUs.

Moving people is fastest. A floater assigned to whichever zone has the longest queue absorbs daily variation, and a supervisor can see the queue on the conveyor without any software. Moving boundaries means redrawing which bays belong to which zone, which only needs a change to the zone map in the warehouse management software if the pick faces sit along a continuous aisle.

• Floater rule: assign one floater per 12 to 15 zone pickers, released to the zone whose utilisation forecast for the next hour exceeds 0.95.
• Boundary moves: shift two or three bays at a time between adjacent zones and re-measure for a week before moving again.
• Re-slotting: move the highest-velocity SKUs from the busiest zone into the next zone downstream, which spreads both lines and visits.
• Break cover: stagger breaks by zone and send the floater into each zone in turn, because a single empty zone stops every tote that needs it.

Parallel zone picking is more forgiving here. A slow zone delays only orders that need it, and other zones keep releasing work, so imbalance shows up as longer consolidation dwell rather than a stopped line. That tolerance is one reason sites with very volatile daily mix, such as fashion e-commerce in sale periods, lean toward parallel picking even when its labour cost is higher.

Consolidation: the hidden cost of parallel zone picking

Parallel picking moves the problem from the zones to the merge point. Every order that touches more than one zone creates totes that must arrive, wait for their siblings and be combined, and the waiting space and labour grow with zones per order.

The merge is usually a put wall, an order-accumulation lane per order, or a pack station that receives all totes for an order. Each needs a rule for what happens when one tote is late or short. Our article on put wall sortation sizes cubbies and covers the orphan-cubby problem when a short pick leaves an order incomplete, which is the main failure of parallel picking.

Parallel zone picking adds consolidation labour equal to total zone totes multiplied by seconds per merge; in a four-zone module with 3,600 orders averaging 2.1 zones, 7,560 totes at 15 seconds each cost 31.5 labour hours, which is the figure pick-and-pass must beat.

Failure trace: a zone-entry scanner fault in a pick-and-pass line

The most expensive pick-and-pass failures are silent: totes flow normally but skip work they needed. The trace below follows a scanner fault on a four-zone line at a US e-commerce site, and the same pattern appears on Indian lines that use fixed barcode readers at zone entries.

1. 14:05, Wednesday: The fixed scanner at zone C entry starts failing reads on glossy tote labels after a cleaning crew moves its mounting bracket by about 2 cm; the conveyor controller treats a no-read as "no lines in this zone" and sends totes straight past.
2. 14:20: Pack stations receive 37 totes with zone C lines missing; packers mark each order short, and the WMS raises 37 short-pick exceptions against zone C locations that show stock.
3. 14:35: The inventory control lead runs cycle counts on six of the locations and finds stock present; the shift manager pulls the scanner read-rate report, which shows zone C at 71% against a normal 99.6%.
4. 14:50: Maintenance re-aims the scanner, and the WES rule changes so that a no-read diverts the tote into the zone for a manual scan instead of bypassing it.
5. 15:30: Team leads return the 37 orders to induction as rework totes; 29 make the 17:00 cutoff and 8 ship the next day, with service credits raised for the late ones.

The fix that mattered was the second one at 14:50. A pick-and-pass line should treat an unreadable label as "unknown", which means stop and check, never as "nothing to pick here". Ask any supplier to demonstrate that behaviour during factory acceptance.

Who owns zone boundaries, routing rules and the labour plan

A zone picking operation crosses engineering, inventory control, operations and controls maintenance. Each record that shapes the flow needs one owner and one approver, or a change in one area, such as re-slotting, quietly unbalances another.

Most routing in a zone line is a fixed decision made by rules, while forecasting and rebalancing benefit from software that recommends without acting. Keep the boundary clear so that a model's suggestion never reroutes a tote by itself.

Commissioning checklist before switching picking methods

A change of zone method is a site project, not a software setting. Test the new flow against real order history and real stoppages, and set pass conditions in numbers before the go-live date is fixed.

For the labour standard behind every number above, our guide to measuring picks per hour explains how to set the per-line time you feed into the model. More warehouse design articles, including multi-level pick modules, are in the logistics and supply chain library.

If you are deciding between pick-and-pass and parallel zone picking for a new or existing site, we can build the workload model from your order history and write the commissioning tests. Share your order data and layout through the request for proposal form.