What is value stream mapping?
Value stream mapping (VSM) is a lean management method, popularized by Mike Rother and John Shook's Learning to See, for visualizing all the steps, inventory and information needed to deliver a product. Unlike a flowchart, a value stream map adds data (cycle times, changeovers, uptime, inventory) and a timeline that compares value-adding time with total lead time. The gap between the two is where improvement lives.
Key value stream calculations
| Metric | Formula | Meaning |
|---|---|---|
| Takt time | Available time per day ÷ customer demand per day | The pace needed to meet demand |
| Inventory days | Inventory units ÷ daily demand | How long parts wait at each point |
| Lead time | Sum of inventory days | Time for one part to travel the value stream |
| Processing time | Sum of cycle times | Value-adding time per unit |
| PCE | Processing time ÷ lead time | Share of lead time that adds value |
Any process whose cycle time exceeds takt time cannot keep up with demand without overtime, extra shifts or added capacity. The tool highlights these in red.
Value stream map symbols used
- Factory icon: supplier and customer.
- Process box with data box: a process step with C/T, C/O, uptime and operators.
- Triangle with "I": inventory waiting between processes.
- Striped arrow: push flow, where material is pushed downstream regardless of need.
- Dashed lines: information flow from production control (schedules, forecasts, orders).
- Timeline ladder: waiting days (top) versus processing time (bottom).
Value stream map example
The example maps a catheter assembly line serving 1,200 units per day over 15 available hours: a takt time of 45 seconds. Tipping, at 52 seconds per unit, exceeds takt and is the bottleneck. About 18,600 units of inventory spread across the line add up to a lead time of 15.5 days, against only 202 seconds of processing time, a process cycle efficiency of about 0.02%.
From current state to future state
- Balance processes to takt; fix the bottleneck first.
- Create continuous flow where processes can be linked.
- Use supermarkets and pull (kanban) where flow is not possible.
- Schedule a single pacemaker process instead of every step.
- Level the production mix and volume (heijunka).
For office and approval processes without physical inventory, use a workflow map. To reduce walking and transport, add a spaghetti diagram.
How to use this Value Stream Mapping tool
- Set customer demand. Click the customer and enter daily demand and available working time. The tool calculates takt time.
- Add the processes. Add each process in order of material flow, and enter cycle time, changeover, uptime and operators.
- Count inventory. Click each inventory triangle and enter the units waiting in front of the process, plus finished goods before shipping.
- Add supplier and information flow. Describe the supplier, delivery frequency, shipping and how production is scheduled.
- Read the timeline. The ladder at the bottom shows waiting days above and processing seconds below, with total lead time and PCE.
- Find the improvements. Look for processes over takt (red), large inventories and low PCE, then design and export your future state.
Frequently asked questions
What is value stream mapping?
Value stream mapping (VSM) is a lean technique that maps the flow of material and information needed to bring a product from supplier to customer. It records process data, inventory and lead time on one page to show where waste and delays are, and to design a better future state.
How do you calculate takt time?
Takt time = available working time per day ÷ customer demand per day. For example, 15 hours (54,000 seconds) for 1,200 units gives a takt time of 45 seconds: the line must complete one unit every 45 seconds to meet demand.
How is lead time calculated on a value stream map?
Inventory between processes is converted into days of customer demand (inventory ÷ daily demand). Lead time is the sum of these inventory days, which is roughly how long a part takes to travel through the value stream.
What is process cycle efficiency (PCE)?
Process cycle efficiency is the value-adding (processing) time divided by the total lead time. In most current-state maps it is well below 1%, showing that parts spend almost all their time waiting.
What data goes in a VSM process box?
Typical process data includes cycle time (C/T), changeover time (C/O), uptime, number of operators, and sometimes batch size, scrap rate and available time. This tool records C/T, C/O, uptime, operators and inventory before each process.
What is the difference between a current state and a future state map?
The current state map shows how the value stream works today. The future state map shows the target design, typically with less inventory, continuous flow, pull systems and processes balanced to takt. Map both and compare lead time and PCE.