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From Excel to a custom application: what about the data

Moving from Excel to a custom application: what to check in your data before a quote, how much history to migrate and what to demand at handover.

by Tsunami Digital Team September 2, 2026

Building a custom application from an Excel sheet or an old program is a project where the data, not the screens, is most of the work. Owners usually ask only once the quote is on the table, yet the answer decides price, timeline and whether the team uses the system.

Why the data is the main cost

Screens are easy to estimate, data is not. A five-year-old sheet holds abbreviations one person understands, customers entered three ways and columns whose meaning changed in 2023. Each case needs a decision before programming.

In a 2017 Harvard Business Review study, 75 managers each reviewed a hundred of their own records: on average 47% had a critical error and only 3% of results were acceptable by the loosest standard.

Four things to check before a quote

Duplicates. The same customer or item entered twice, slightly differently.
The same data in two formats. Dates as text and as dates, prices with and without VAT, kilograms and pieces mixed.
Rules that live in people’s heads. “If the customer is from Istria, the discount is applied by hand.” Unwritten rules are the most expensive surprise.
How much history you need. Statutory records and open invoices, yes; closed 2019 quotes, usually not.

Send those notes with your enquiry when you commission custom software.

Migrate history or start from a cut-off date

Migrating history cleans and imports every record: slower and dearer, but complete from day one. A cut-off date imports only open items and master data (customers, items, prices); the old sheet becomes a read-only archive.

For most small projects the second route wins, by the logic of small-scope software projects. Either way, lock the old system after the switch; while both accept entries, the old one wins.

Leaving a real legacy program is harder; legacy system migration: rebuild or keep works through it.

What to demand at handover

One-click export of all data in an open format (CSV or Excel), without the agency.
A documented data structure: which table holds what, and how they link.
An import log: records imported, rejected, and why.

Without those, the data stays trapped. What else ownership includes is in who owns the custom software you paid for.

When you do not need a new application

If one person uses the sheet, formulas hold and nobody asks which version is current, stay on it. The signs you have outgrown it are in internal tools beyond spreadsheets; the payback calculation is in custom software development for small business.

Frequently Asked Questions

Must we clean the data ourselves first? No, but you must know what is messy. Cleaning is part of the project if scoped from the start.

Will we lose data during migration? Not if the import keeps a log of rejected rows and the old sheet stays archived.

Can the data stay in Excel, with the application only reading it? As a transition, yes. Permanently, no: two sources of truth bring back every problem you wanted solved.

What about data on paper and in email? Entered by hand or extracted automatically; the second route is in data collection for business.

How much does migration add? On the state of the data, not the amount: a hundred thousand clean rows cost less than a thousand messy ones.

Have a sheet everyone uses and nobody trusts?

Send us its structure, without confidential data, and we will tell you what migration would cover.

Reach out at [email protected] or via the form on our homepage.

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