What does Inventory Dollar-Days (IDD) measure?

Inventory Dollar-Days (IDD) measures how much cash is tied up in inventory and for how long. In practice, you take the value or cost of inventory and multiply it by the number of days it has been sitting. In a steel shop, that might be part-finished frames, cut profiles or bought-in items that have been released but are not moving, showing you where money is aging on the floor or in the rack.

What does Throughput Dollar-Days (TDD) measure?

Throughput Dollar-Days (TDD) measures how much throughput value is late and for how long. You multiply the value of a job (often contribution margin or job price) by how many days it is late versus the committed ship date. This turns late deliveries into a financial priority list, so you know which overdue job is hurting you most right now instead of treating all late orders as equal.

Why do TDD and IDD matter for EZIIL’s ICP of custom fab and job shops?

Custom fabrication is a constant tradeoff between releasing work early 'just in case' (which increases WIP and IDD) and keeping WIP under control (which reduces IDD but demands better planning). TDD and IDD help you answer two key questions: Which late job hurts us most right now (TDD)? And where is our cash stuck aging on the floor or in the rack (IDD)? For EZIIL’s ICP of small and mid-sized job shops, these metrics support better decisions on what to release, what to accelerate and what to park.

How do you calculate IDD for WIP aging in a steel shop?

A simple way to calculate IDD for work-in-progress is: 1) assign a value to each open job, such as material cost or total job value, 2) measure how many days it has been released to production, and 3) multiply value by days in WIP. Sorting these scores from highest to lowest gives you an 'aging money pile' list. This shows which jobs are tying up the most cash for the longest time and should be finished or shipped first to free up money.

How do you calculate TDD to prioritise late jobs?

To calculate TDD for late jobs, you can: 1) assign a value such as contribution margin or job price to each order, 2) measure how many days it is late versus the committed ship date, and 3) multiply the value by the days late. The highest TDD scores represent the most painful late jobs in financial terms. This helps you decide which projects to push next based on impact, not on who is shouting loudest.

How can TDD and IDD be used without going full Theory of Constraints?

You do not need to implement the full Theory of Constraints to benefit from TDD and IDD. A pragmatic approach is to calculate IDD for aging WIP using job value and days since release, and TDD for late jobs using job value and days late. Review both lists regularly, focus on the top 10 items, remove blockers and rebalance labour. This gives you most of the decision-making benefit without complex models or big consulting projects.

What best practices support good TDD/IDD performance in a steel job shop?

Good TDD/IDD practice includes setting WIP limits so you release work when there is capacity, not just when you feel nervous; kitting materials before release to avoid half-started jobs that age on the floor; using buffer-based urgency to decide what must move today to protect ship dates; and reviewing the top 10 'aging jobs' weekly to remove routing mistakes and rebalance labour. These habits keep both lateness and stuck inventory under control.

How does EZIIL help operationalise TDD/IDD thinking in daily planning?

EZIIL makes it easier to apply TDD/IDD thinking by giving you one live view of all active projects, their values, statuses and deadlines. You can see which jobs are approaching or past their promised dates, how work is distributed across departments and where jobs are stuck. Comparing planned vs actual hours highlights early warning signals: jobs that are burning hours fast or not moving at all, which typically become TDD or IDD problems later. Real status updates in EZIIL also reduce the 'inventory you forgot you had' problem, because every job’s position in the flow is visible.

How can EZIIL be used to limit WIP and reduce IDD in a custom fab shop?

With EZIIL, you can see how loaded each department is and choose to release new jobs only when there is real capacity, which naturally limits WIP and reduces IDD. Planning views make it clear which projects are in engineering, cutting, welding or finishing, so you avoid flooding a bottleneck with extra work. Because EZIIL links materials, tasks and deadlines in one place, it also supports kitting before release, helping you avoid half-started jobs that sit for days and accumulate Inventory Dollar-Days.

How do TDD/IDD metrics improve cash flow and on-time delivery?

By focusing on TDD, you prioritise finishing the late jobs with the biggest financial impact, which improves on-time delivery where it matters most to revenue and customer relationships. By monitoring IDD, you identify where cash is stuck in aging WIP and push those jobs to completion or shipment to free up money. Together, TDD and IDD shift the planning conversation from 'what can we start' to 'what must we finish', which is where cash flow and OTD actually improve.

Who should monitor TDD and IDD in a steel fabrication company?

In most small and mid-sized steel shops, TDD and IDD are shared metrics across roles. Owners and general managers look at them to understand where money and risk are concentrated. Planners and production managers use them to set daily and weekly priorities. Project managers and sales can use the same information from EZIIL to communicate realistic dates to customers. Because EZIIL centralises project values, deadlines and status, everyone can look at the same facts instead of relying on different spreadsheets or individual memory.

Throughput Dollar-Days/Inventory Dollar-Days (TDD/IDD)

Q: What are TDD and IDD in a custom steel fabrication shop?

TDD (Throughput Dollar-Days) and IDD (Inventory Dollar-Days) are Theory of Constraints style metrics that combine money and time to show the real impact of lateness and inventory. Instead of treating every late job or piece of WIP the same, they weight each one by its financial value, helping custom fab and job shops focus on the jobs and inventory that hurt the most.

What is Throughput Dollar-Days/Inventory Dollar-Days (TDD/IDD) in steel fabrication?

TDD (Throughput Dollar-Days) and IDD (Inventory Dollar-Days) are Theory of Constraints (TOC) style metrics that combine money × time to show the real pain of lateness and inventory.

Instead of treating every late job the same, they weight impact by dollars.

IDD measures how much cash is tied up in inventory and for how long. It’s commonly described as the value (or cost) of inventory × days it’s been sitting.
TDD measures how much throughput value is “late” and for how long. It’s a way to quantify late deliveries weighted by their economic impact.

Why Throughput Dollar-Days/Inventory Dollar-Days matter for custom steel fabricators

Custom fabrication is basically a constant tradeoff between:

releasing work early “just in case” (which increases WIP + IDD)
keeping WIP under control (which reduces IDD but demands better planning)

TDD/IDD help you answer two questions quickly:

Which late job hurts us most right now? (TDD)
Where is our cash stuck aging on the floor or in the rack? (IDD)

How to use to use TDD and IDD in a steel shop

You can adapt the concept with shop-friendly inputs:

IDD for WIP aging

Assign a value (material cost, or “job value”) to each open job
Multiply by days since release to production
Sort descending: that’s your “aging money pile”

TDD for priority

Assign a value (contribution margin, or job price)
Multiply by days late vs committed ship date
Highest score gets attention first (not whoever shouts loudest)

Best practices for TDD and IDD

Set WIP limits (release work when there’s capacity, not when you feel nervous)
Kit materials before release to avoid half-started jobs aging on the floor
Use buffer-based urgency (what must move today to protect ship dates)
Review top 10 “aging jobs” weekly: remove blockers, fix routing mistakes, rebalance labor

How EZIIL helps with TDD and IDD

If you’re using EZIIL’s steel fabrication software you can:

See all active projects and deadlines in one place (so urgency is shared, not tribal knowledge)
Assign and track work across departments, then spot where jobs are stuck before they become “surprise late”
Compare planned vs actual to catch early warning signals (a job burning hours fast usually becomes a TDD problem later)
Use real status updates to reduce the “inventory you forgot you had” problem

Related resources

Eziil

Throughput Dollar-Days/Inventory Dollar-Days (TDD/IDD)

What is Throughput Dollar-Days/Inventory Dollar-Days (TDD/IDD) in steel fabrication?

Why Throughput Dollar-Days/Inventory Dollar-Days matter for custom steel fabricators

How to use to use TDD and IDD in a steel shop

Best practices for TDD and IDD

How EZIIL helps with TDD and IDD

Related resources

Metal Fabrication ERP Guide for MTO Shops: When to choose EZIIL over a generic ERP

Introducing Eziil Starter—Your first step toward smarter metal fabrication management

VMT boosts their engineers’ productivity by 40% with Eziil BOM

Get Started

Our Company

Resources