Time study: how to calculate the standard time of a task

In one line

A standard time is not how long the operator took during the stopwatch timing: it is how long a qualified person takes at a normal pace, plus the time that person needs to rest, attend to personal needs and absorb the unavoidable delays.

What it is

Time study is the work measurement technique that sets an allowed time for a specific task, by measuring the work content of the defined method and accounting for fatigue, personal delays and unavoidable delays. It is a tool that originated with Taylor in the late 19th century and is still in use because it is the basis of almost everything else: capacity, costs, line balancing, incentives.

The calculation chain has three links, and each one corrects a different problem in the previous one:

• Observed time (OT): what the stopwatch measures, averaged over several cycles.
• Normal time (NT): the observed time adjusted by the pace rating. If the operator worked faster than a normal pace, the time is corrected upward; if the operator worked slower, downward. It is calculated as NT = OT × (rating ÷ 100), where 100 is the normal pace.
• Standard time (ST): the normal time plus the allowances. It is calculated as ST = NT × (1 + allowance).

What it is for

Without the rating step, the standard punishes the fast operator and rewards the slow one: every person has a different natural pace, and what you want to set is a value that works for anyone. The UNGS book puts it with a simple idea: the product of the observed time and the observed rating, divided by the normal rating, has to yield a constant. That "universalized" value is the normalized time, the time taken to perform an element at a normal pace. According to the ILO methodology the book cites, that rating is recorded while the element is being performed and before it ends, so that it is not contaminated by the time the stopwatch shows.

Allowances exist for a practical reason: nobody works the 480 minutes of a shift at a normal pace without pauses. Niebel groups them into personal needs, basic fatigue and unavoidable delays, and notes that on average an allowance of 15% is used for manual elements and around 10% for machine elements. The UNGS book also distinguishes between constant allowances (personal needs and basic fatigue) and special allowances, which depend on particular conditions of the workstation.

How to apply it

1. Define the method before measuring. The standard measures the method under study; if the method changes afterward, the standard is no longer valid.
2. Divide the task into elements with a clear start and end, so that each one can be timed separately.
3. Time several cycles and record, along with each reading, the rating of the operator's pace.
4. Discard anomalous readings (a defective part, an interruption unrelated to the task) and average the valid ones.
5. Check the record. Niebel requires the error between the total stopwatch time and the actual elapsed time to be less than 2%; if it exceeds that, the study is repeated.
6. Calculate the normal time of each element and apply the allowance to obtain the standard time. The sum of the standards of the elements gives the standard of the task.

There are two ways to apply the allowance, and it is worth knowing which one each company uses. The most common adds it as a percentage of productive time: with a 10% allowance, the multiplier is 1.1. The second calculates it over the full workday: the multiplier becomes 100 ÷ (100 − 10) = 1.11. In a 480-minute shift, the first leaves 436 productive minutes and 44 of rest; the second assigns 48 of rest. The difference is small, but it is a classic source of argument when comparing standards between plants.

Real example

Niebel and Freivalds work through the case of a drill press operator. The book's data: a 480-minute workday, 420 units drilled, a worked fraction of 85%, an average pace rating of 110% and allowances of 15%. An important note: in this example the book obtains the worked fraction through work sampling, not with a stopwatch, but from the observed time onward the calculation is exactly the same.

• Observed time: 480 × 0.85 ÷ 420 = 0.971 minutes per unit.
• Normal time: 0.971 × 110 ÷ 100 = 1.069 minutes.
• Standard time: 1.069 × 1.15 = 1.229 minutes, or about 49 pieces per hour.

The operator worked at a better than normal pace (110%), so the actual time of 0.971 minutes was corrected upward: the standard is more demanding than what the operator actually took.

A second example, from the UNGS book, illustrates the step of cleaning the data. A series of readings was recorded for one element; one of them, of 49 hundredths of a minute, also included the observation of a defective part and was discarded as invalid. The 23 valid readings added up to 611 hundredths, an average of 26.6, and 0.27 minutes was adopted as the normalized time of the element.

Calculate your standard time

Template to use

A time study sheet needs only these columns. One row is filled in for each element of the task:

Element	Readings (min)	Rating (%)	Average OT	NT = OT × rating ÷ 100	Allowance (%)	ST = NT × (1 + allowance)
1. (describe the element)	(one reading per cycle)	(observed pace)
2. (describe the element)
Task total						Sum of the ST

So you do not have to build it by hand, there is a time observation sheet in Excel with these same calculations (average time, rating, normal time, allowance and standard time), plus the check of the recording error.

At the bottom of the sheet it is a good idea to note the method studied, the date, who timed it, the operator observed and the recording error, so that the standard can be audited later.

Benefits

• It gives a defensible number for calculating capacity, cost per piece and the load of each station, instead of eyeball estimates.
• It is the direct input for line balancing and for the cycle time that is compared against the takt time.
• It forces you to define the method before measuring, which by itself often uncovers improvements.

Limitations to keep in mind

• Pace rating is a judgment by the analyst, and it is the most disputed part of the method. An analyst with little training introduces error and distrust into the result.
• A standard is valid for the method in which it was measured. If the method, the tool or the layout changes, you have to study it again.
• The stopwatch is not practical for very long or very non-repetitive tasks; other techniques exist for that, such as work sampling or predetermined time systems.
• A standard applied without explaining to the team how it was calculated tends to generate rejection, even if it is well done.

In summary

The standard time is built in three steps: measure (observed time), correct for pace (normal time) and add the allowances (standard time). What matters is not the formula, which is a multiplication, but the discipline around it: a defined method, clean readings, trained rating and allowances agreed in advance.