How to Choose a Commercial Dishwasher: Sizing for Your Volume

How to Choose a Commercial Dishwasher: Sizing for Your Volume

One of the most common and costly mistakes catering operators make when buying a commercial dishwasher is under-specifying the machine for their volume. A machine that cannot keep up with service creates bottlenecks, forces staff to re-wash items, and pushes the business towards using disposables at exactly the wrong moment. Over-specifying wastes capital and ongoing energy costs.

This guide gives you a straightforward, step-by-step method for calculating the right dishwasher capacity for your operation. Work through each step, apply the worked example, and you will arrive at a clear specification before you browse the available models.

Browse our full range at commercial dishwashers once you have completed your calculation.

Step 1: Calculate Your Covers Per Day

Start with the total number of covers your operation serves across all services in a typical busy day. Be honest and use your peak-trading day, not your average. A machine that copes on a quiet Tuesday but fails on a Saturday evening is the wrong machine.

Separate your covers by service type if they differ significantly:

• Breakfast covers (typically lighter loads — fewer pots, more crockery)
• Lunch covers
• Dinner covers (typically heaviest load — full crockery, cutlery, serving dishes)

Write down your total covers for your busiest full day.

Step 2: Calculate Dishes Per Service

Convert covers into individual items to be washed. The number of washable items generated per cover varies by service type. Use the following estimates as a starting point:

• Café / light meal: 4 to 6 items per cover (cup, saucer or mug, plate, side plate, one or two pieces of cutlery)
• Casual lunch: 6 to 9 items per cover (starter bowl or side plate, main plate, two to three pieces of cutlery, drinking glass, coffee cup)
• Full restaurant dinner service: 10 to 16 items per cover (bread plate, starter, main, dessert, multiple cutlery pieces, wine glass, water glass, coffee cup)
• Add serving dishes and pots: For every 10 covers, estimate 3 to 5 additional serving items (sauce pots, sharing boards, vegetable dishes, serving spoons)

Multiply your covers by your items per cover to get your total items per service. Then identify your single busiest service — this is the load your machine must handle within a defined time window.

Step 3: Determine Your Time Window

A dishwasher does not need to wash everything generated in a full day within a single hour. It needs to process items fast enough to keep up with the service as it unfolds and ensure clean items are available for re-use.

For most restaurants, the practical time window is the duration of the busiest service. Identify:

• How long does your busiest service last? (Often 90 minutes to 2.5 hours for a dinner service)
• Are items used once and cleared, or are tables turned during the service?
• Do you have sufficient backup crockery and glassware to buffer a slower throughput rate?

If your crockery inventory is small and tables turn rapidly, your effective time window is short — perhaps 60 minutes. If you have three full sets of crockery for each table and a slower turnover, you have more flexibility.

Step 4: Calculate Racks Per Hour Required

Now convert items per service into racks per hour needed. Use these standard rack capacities as your reference:

• 400 mm × 400 mm rack (standard undercounter): 25 dinner plates, or 25 pint glasses, or 36 side plates, or 120 pieces of cutlery with inserts
• 500 mm × 500 mm rack (pass-through and hood-type): Approximately 40% more capacity than a 400 mm rack

Calculate the number of racks required to process your busiest service load. Divide by your available time window in hours to arrive at your required racks-per-hour figure.

Then add a 20% safety margin. A machine running at 100% of its rated capacity every service will wear faster and leave no buffer if service runs over or if an unusually large party arrives.

Step 5: Match to Machine Type

Use your required racks-per-hour figure to identify the appropriate machine type:

• Up to 20 racks per hour: Basic undercounter dishwasher
• 20 to 40 racks per hour: Quality undercounter dishwasher
• 40 to 60 racks per hour: Hood-type or entry-level pass-through machine
• 60 to 100 racks per hour: Mid-range pass-through machine
• 100 to 150 racks per hour: High-capacity pass-through machine
• 150+ racks per hour: Conveyor dishwasher

Worked Example: A 60-Cover Restaurant

Let us apply the method to a realistic scenario. The Northampton Arms is a 60-cover restaurant serving a lunch service (11:30 to 14:30) and a dinner service (18:00 to 22:00). Dinner is the busiest service.

Step 1: Covers

60 covers at dinner. Tables turn once (no second sitting). Total dinner covers: 60.

Step 2: Items Per Cover

The restaurant serves a three-course dinner. Using the full restaurant dinner estimate:

• 12 items per cover (bread plate, starter bowl, main plate, side plate, dessert plate, fish fork, dinner fork, knife, dessert spoon, wine glass, water glass, coffee cup)
• Total crockery and glassware: 60 × 12 = 720 items
• Serving items (at 4 per 10 covers): 24 additional items
• Kitchen pots and pans: estimate 30 items
• Total items: approximately 774

Step 3: Time Window

Service runs from 18:00 to 22:00, but the majority of covers sit down between 19:00 and 20:30. The practical wash window is 3 hours (items start coming back from tables at 19:15, machine should be clear by 22:30).

Step 4: Racks Per Hour

One 400 mm rack holds approximately 25 plates. 774 items across a range of plate sizes and glasses equates to roughly 28 to 32 racks total for the service.

Divided across a 3-hour window: 32 ÷ 3 = approximately 11 racks per hour. Adding 20% safety margin: 14 racks per hour.

Step 5: Machine Type

14 racks per hour is comfortably within the capability of a quality undercounter dishwasher rated at 25 to 30 racks per hour. The Northampton Arms does not need a pass-through machine for dinner service alone.

However, if the restaurant adds a lunch service of similar scale, the total daily load doubles, and the machine runs for six or more hours. In this case, a more robust undercounter machine or a small hood-type machine would be advisable for longevity.

Additional Factors to Consider

Space Constraints

Measure your available floor space accurately before specifying a machine. Do not forget to account for:

• Clearance for the door or hood to open fully
• Space for inlet and outlet tables (essential for efficient operation)
• Access for cleaning and maintenance
• Ventilation clearance above the machine

Standard undercounter machines are typically 600 mm wide × 600 mm deep × 820 mm high (to fit under a 900 mm worktop). Pass-through machines vary significantly — check dimensions carefully, including height with the hood open.

Water Hardness

Water hardness is a major factor in UK dishwasher performance and machine longevity. Water hardness is measured in parts per million (ppm) of calcium carbonate:

• Soft: 0 to 100 ppm (Scotland, West Wales, South West England)
• Moderately hard: 100 to 200 ppm (Northern England, Midlands)
• Hard: 200 to 300 ppm (East Midlands, East England, South East)
• Very hard: 300 ppm+ (parts of London, Kent, Essex)

Northampton sits in a hard water area. Any commercial dishwasher installed here should include an integral water softener, or an external in-line softener must be fitted. Without softening, heating elements scale up within months, rinse quality deteriorates, and machine life is significantly shortened.

Check your local water hardness using your water company's online tool or contact Caterzone for advice specific to your location.

Noise Levels

Noise is rarely considered during machine selection but becomes a significant operational issue in open-plan kitchens, open-kitchen restaurant concepts, or venues where the wash station is close to the dining area.

• Standard commercial dishwashers: 62 to 70 dB(A) during operation
• Quiet-rated machines: 55 to 60 dB(A)

If noise is a concern, look specifically for machines marketed as low-noise models. The reduction in acoustic output is achieved through insulated double walls, vibration-dampened pumps, and improved door seals. There is typically a cost premium of 10 to 20% over comparable standard machines.

Warranty and Service Coverage

A commercial dishwasher working in a busy kitchen is a critical piece of infrastructure. If it fails during service, the consequences are immediate and significant. Warranty and service response time should be part of your purchase decision.

• Standard warranty: 12 months parts and labour — acceptable for lower-volume operations
• Extended warranty: 24 to 36 months — advisable for high-volume primary machines
• Service response: Check the manufacturer or supplier can provide same-day or next-day service call-out across the UK. Many smaller distributors cannot.
• Spare parts availability: Confirm spare parts will be available for the full expected life of the machine (minimum five to seven years for a primary machine)

Caterzone supplies machines from manufacturers with established UK service networks. We can advise on service coverage in your area at the point of purchase.

Summary: Sizing Checklist

1. Calculate your busiest day's covers by service
2. Multiply by items per cover to get total items per service
3. Identify your practical wash window (hours)
4. Convert to racks per hour and add 20% safety margin
5. Match to machine type using the racks-per-hour guide
6. Check space constraints — measure accurately
7. Confirm local water hardness and softener requirement
8. Assess noise requirements if relevant
9. Check warranty terms and service network coverage

If you work through these steps and are still unsure, the Caterzone team is available to help. Browse commercial dishwashers or contact us to discuss your specific situation.