Build an Opex Planning Model
Build an Opex Planning Model
This guide explains how to build a structured Opex planning model in Model Reef using Opex and Staff variables, categories, and drivers. The focus is on building a clear, scalable cost view that ties into P&L, Cashflow and valuation.
What you will build
A category based Opex layout (for example marketing, rent, technology, overhead).
Opex variables that can be fixed, variable or driver based.
A forecast that flows into:
Operating Expenses in P&L.
Payments to Suppliers in Cashflow.
Opex lines in the Cash Waterfall.
A structure you can adapt for budgets and scenarios.
Define your Opex categories
Start with a simple but useful category structure, for example:
Marketing and advertising.
Technology and software.
Rent and premises.
Professional services.
Travel and entertainment.
General and administrative.
These should map cleanly to how you want to see Opex in reports and dashboards.
Create base Opex variables
For each category, create one or more Opex variables:
Name them clearly, such as:
Opex - Marketing - Paid Ads.Opex - Technology - SaaS Tools.Opex - Rent - Head Office.
Assign categories and sub categories consistent with your hierarchy.
Set the basic amount pattern:
Fixed monthly or annual values.
Scheduled one offs.
Simple escalation over time.
This creates a foundation you can refine with drivers.
Introduce drivers for variable Opex
Some Opex behaves as a function of activity or revenue, for example:
Marketing spend as a percentage of revenue.
Payment processing fees linked to transaction volume.
Customer support costs linked to active users.
To model these:
Create drivers in the Data Library for the underlying activity, for example:
Driver - Marketing ratio to Revenue.Driver - Payment fee rate.
In the relevant Opex variables, use formulas that reference these drivers and revenue or volume variables, for example:
Opex - Marketing - Paid Ads = Revenue × Marketing ratio driver.
Check the resulting Opex pattern as revenue changes.
This ensures that Opex scales coherently with the business.
Add timing and seasonality
For Opex with seasonality or irregular patterns:
Use scheduling or presets to spike spend in relevant periods, such as:
Annual insurance premiums.
Periodic marketing campaigns.
Project based consulting fees.
In each Opex variable, configure:
Start date and end date.
Frequency and schedule.
Any payment delays if invoices are paid after services are delivered.
These rules flow through to P&L and Cashflow automatically.
Review Opex profile and ratios
Open the P&L and review:
Total Opex by category.
Opex as a percentage of revenue.
Key ratios such as marketing spend to revenue or technology spend per customer.
You can build charts that show:
Opex mix over time.
Opex per unit (for example per user or per store).
Opex comparisons across branches or divisions.
This helps you understand cost structure and opportunities for efficiency.
Build Opex planning scenarios
To plan for different cost strategies, create separate models or scenario copies, for example:
Model - Opex - Base Plan.Model - Opex - Cost Control.Model - Opex - Growth Investment.
In each:
Adjust Opex drivers and escalation rates.
Change variable Opex ratios to revenue.
Introduce or remove specific spend initiatives.
Compare EBITDA, cash and valuation across these Opex strategy models.
Check your work
Opex categories reflect how you want to manage and report costs.
Fixed and variable Opex are clearly separated where helpful.
Activity linked Opex responds correctly to revenue or other drivers.
P&L and Cash Waterfall show a believable cost trajectory.
Troubleshooting
Related guides
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