How to Lookup Lowest Monday Tide in Excel

Why This Task Matters in Excel

Imagine you work for a coastal engineering firm, a shipping company, or a tourism board that schedules boat tours. In all these roles, understanding tide levels on specific weekdays is critical. You might need to decide when dredging equipment can safely operate, schedule ferry departures to avoid grounding, or plan walking tours over tidal flats. Mondays are especially important in many industries because they mark the first operational day of the week: dredging crews return from the weekend, cruise ships restart passenger embarkations, and scientific field teams kick off their sampling campaigns.

Looking up the lowest Monday tide quickly—instead of scanning a printed tide table—saves enormous time, prevents costly mis-scheduling, and reduces safety risks. A single missed shallow tide can damage equipment, delay shipments, and incur severe penalties. Having a dynamic Excel solution means you can drop in new tide data every month or year and instantly know when the most favorable low tide occurs on Mondays.

Excel is perfect for this because it pairs powerful date-handling functions with flexible lookup tools. You can filter by weekday, compute minima, and retrieve the full record (date, time, height) in a single step. Mastering this workflow also builds broader skills: date arithmetic, conditional aggregation, dynamic arrays, and data validation. If you do not know how to do it, you might rely on manual filtering or pivot tables every time the dataset updates—an error-prone and time-consuming approach. By learning it once, you unlock a repeatable process you can redeploy across other weekday-based analytics, such as “highest Friday sales” or “average Wednesday temperature.”

Best Excel Approach

The most efficient modern solution combines three dynamic-array functions: FILTER, SORTBY, and INDEX (or TAKE) wrapped inside LET for readability. This method keeps everything inside one formula, requires no helper columns, and works in Microsoft 365 or Excel 2021+.

Logic overview

1. Evaluate which rows are Mondays.
2. Filter the original dataset down to those rows.
3. Sort the remaining rows by tide height (ascending).
4. Return the first row (the lowest tide).

Formula (assume dates in [A2:A1000] and tide heights in [B2:B1000]):

=LET(
     dates, A2:A1000,
     heights, B2:B1000,
     monRows, FILTER(SEQUENCE(ROWS(dates)), WEEKDAY(dates, 2)=1),
     result, TAKE(SORTBY(CHOOSE({1,2}, INDEX(dates, monRows), INDEX(heights, monRows)), INDEX(heights, monRows), 1), 1),
     result
)

Key points

• WEEKDAY(dates,2) returns 1 for Monday, 2 for Tuesday … 7 for Sunday.
• CHOOSE([1,2], …) rebuilds a two-column array so you can keep both the date and the height together for later retrieval.
• SORTBY arranges the filtered table by height in ascending order.
• TAKE(…,1) extracts the first (lowest) row.

When to use this method

• Your organization runs Microsoft 365 or Excel 2021+.
• You want a single cell that instantly refreshes when you paste new tidal data.
• Dataset size is moderate to large (hundreds to hundreds of thousands of rows) and you prefer avoid helper columns.

Alternative classic approach (compatible with Excel 2016 or earlier):

=INDEX(A2:B1000, MATCH(MIN(IF(WEEKDAY(A2:A1000,2)=1, B2:B1000)), B2:B1000, 0), 0)

This is an array formula; confirm with Ctrl + Shift + Enter in older Excel. It finds the minimum Monday tide, then matches that height back to the row.

Parameters and Inputs

• dates (Range) – A contiguous column containing Excel serial dates or date-time values. They must be valid Excel dates, not text.
• heights (Range) – A column of numeric tide levels (meters, feet, or centimeters). All cells should be numeric so MIN and SORTBY work correctly.
• Return_type (Number) – In WEEKDAY(...,2) the “2” parameter dictates Monday=1. Changing this number changes weekday mapping; keep at 2 for intuitive Monday=1 logic.
• Optional extra columns – You can add tide state (Low, High), location, or barometric pressure. Add them inside CHOOSE so they flow through the same pipeline.

Data preparation

• Remove blanks: Empty cells break MIN and may shift indexes.
• Ensure dates and heights are aligned row-by-row.
• Validate no duplicates if you plan to pull ONLY one row; duplicates can be handled with TAKE(…,1) or FILTER to return multiple rows.

Edge cases

• Missing Mondays – FILTER returns a #CALC! error. Surround the formula with IFERROR to display “No Monday in data.”
• Non-numeric heights – Convert “–” placeholders to blanks or NA() sentinel values.
• Multiple lowest ties (exact same height) – Decide whether you need the earliest or latest date; use SORTBY(dates, 1) as secondary key.

Step-by-Step Examples

Example 1: Basic Scenario

Suppose you have a simple sheet named “Tides” with the following sample data:

[Sheet area]

• Column A (DateTime): 03-Jan-2022 02:15, 03-Jan-2022 14:25, 04-Jan-2022 03:10, 10-Jan-2022 01:55, 10-Jan-2022 14:09 …
• Column B (Tide m): 1.2, 2.9, 1.3, 0.8, 2.7 …

Goal: Retrieve the lowest Monday tide height and its date.

Step 1: Enter the modern formula in D2:

=LET(
  dates, A2:A100,
  heights, B2:B100,
  monday, WEEKDAY(dates,2)=1,
  dataMon, FILTER(CHOOSE({1,2}, dates, heights), monday),
  TAKE(SORTBY(dataMon, INDEX(dataMon,,2), 1), 1)
)

Step 2: Press Enter (Excel 365 only). The single cell spills two values:

• 10-Jan-2022 01:55
• 0.8 m

Explanation

• monday returns a Boolean array marking rows where Monday is true.
• FILTER keeps only those rows.
• SORTBY arranges them by the tide column (second column) in ascending order.
• TAKE picks the top row, which is the minimum.

Common variations

• Return only the height: wrap the LET with INDEX(… ,2).
• Return the nth-lowest Monday tide: change TAKE(…,1) to TAKE(…,n).

Troubleshooting

• If you see #CALC!, double-check that at least one record falls on Monday.
• If you get #VALUE!, ensure column B has numeric entries (no “N/A”).

Example 2: Real-World Application

Scenario: A harbor master maintains a monthly log of four daily tide readings (low and high) for three docks. The sheet has 3 600 rows:

Column A: DateTime
Column B: Dock (text)
Column C: Tide ft (numeric)

Requirement: For Dock “West-Pier” find the date and depth of the lowest Monday tide so dredging can be scheduled.

Data setup

• Dock column may contain “West-Pier”, “Central-Pier”, “East-Pier”.
• Dates cover the entire month.

Step-by-step

1. In G1 enter the target dock name: West-Pier.
2. In G2 enter the advanced formula:

=LET(
    dates, A2:A3601,
    docks, B2:B3601,
    heights, C2:C3601,
    criteria, (docks=G1)*(WEEKDAY(dates,2)=1),
    selected, FILTER(CHOOSE({1,2,3}, dates, docks, heights), criteria),
    TAKE(SORTBY(selected, INDEX(selected,,3), 1), 1)
)

3. Result spills three fields: date-time, dock, tide level.

Why this solves the business problem

• Harbor staff can adjust equipment based on the exact time the water is shallowest at the target dock on Mondays.
• The formula updates automatically when next month’s CSV is pasted.

Integration with other Excel features

• Conditional formatting can highlight the returned date in the master list.
• A Power Query step could append multiple months; the formula keeps working.

Performance considerations

• Even at 100 000 rows the LET-FILTER-SORTBY chain recalculates almost instantly on modern hardware because calculations run on Microsoft’s new threading engine.

Example 3: Advanced Technique

Edge case: You have ten years of hourly tide data (about 87 600 rows per year = 876 000 rows) housed in a structured Table called tblTides with fields DateTime and Height. You need a dashboard that shows:
a) Lowest Monday tide in the selected year (slicer)
b) The next two lowest Monday tides (for contingency)
c) A chart referencing those three dates

Step-by-step

1. Insert a slicer connected to tblTides[DateTime] and set it to “Report Year.”
2. Configure cell K1 to store the selected year with:

=YEAR(MIN(tblTides[DateTime]))

3. Use a dynamic named range for the year filter:

=FILTER(tblTides, YEAR(tblTides[DateTime])=K1)

4. Combine with Monday filter and retrieve the three lowest tides:

=LET(
  data, FILTER(tblTides, YEAR(tblTides[DateTime])=K1),
  mondays, FILTER(data, WEEKDAY(INDEX(data,,1),2)=1),
  lowest3, TAKE(SORTBY(mondays, INDEX(mondays,,2), 1), 3),
  lowest3
)

5. Link a small line chart to the DateTime and Height columns inside lowest3. The chart auto-updates when the slicer year changes.

Advanced tips

• Use OFFSET and XLOOKUP to dynamically supply chart ranges if your Excel version is pre-365.
• For performance, store the decade of data in Power Pivot and create a DAX measure for “Lowest Monday Tide.”

Error handling

• Wrap the LET in IFERROR(…, \"No Monday data\") to cover leap years where January 1 might not introduce a Monday in the filtered year.

Professional best practice

• Document the formula in a hidden sheet using the FORMULATEXT function for auditability.

Tips and Best Practices

1. Name your ranges (e.g., rngDates, rngTides) to make LET formulas readable and reduce maintenance errors.
2. Use return_type = 2 in WEEKDAY so Monday=1; this aligns intuitively with “first day of the workweek.”
3. Store data in Excel Tables; FILTER and SORTBY automatically resize when you append new CSV files.
4. For dashboards, return the full record (date and tide) then reference with structured column syntax—charts love stable spilled ranges.
5. Surround your master formula with IFERROR early in development; unexpected blanks or text will otherwise surface errors at stakeholders’ desks.
6. Keep an eye on time zones; tide tables often ship as local time. If you import UTC, adjust with TIME or DATEVALUE so Monday stays Monday.

Common Mistakes to Avoid

1. Mixing text dates and real dates: Excel treats “2022-01-10” (text) differently from a true serial date, breaking WEEKDAY. Convert with DATEVALUE.
2. Forgetting return_type: Using WEEKDAY(date) defaults to Sunday=1; your Monday filter becomes Sunday filter and yields the wrong tides.
3. Matching the minimum value on the entire dataset: If you forget the weekday filter, MIN returns the absolute lowest tide, not Monday’s lowest. Always apply criteria first.
4. Duplicate minima not handled: If two Monday tides share the same lowest value, INDEX-MATCH returns the first match only. Decide whether that suits your operational needs or switch to FILTER to capture ties.
5. Array formula not confirmed (legacy Excel): Users press Enter instead of Ctrl + Shift + Enter, causing #N/A results. Clarify the confirmation keys in shared instructions.

Alternative Methods

Below is a side-by-side comparison of four viable techniques.

Pros and cons

• LET-FILTER-SORTBY is compact, self-contained, and dynamic but requires the newest Excel.
• MINIFS with a helper “Weekday” column is simple to audit and works in 2019 but clutters the sheet.
• AGGREGATE avoids helpers yet can be hard to read and lacks dynamic spill of extra columns.
• PivotTables are user-friendly for non-formula experts but need manual refresh or VBA to update instantly.

Migration strategy
Start with a PivotTable if you are exploring the data interactively. Once you settle on the metric, translate the logic into a LET formula for full automation.

FAQ

When should I use this approach?

Use the LET-FILTER-SORTBY stack when you need a one-cell answer that automatically refreshes as new tide logs are pasted and when you run Excel 365 or 2021.

Can this work across multiple sheets?

Yes. Qualify ranges with sheet names (e.g., \'January\'!A2:A1000) or place all month tables into a single Table that spans sheets via Power Query. Dynamic arrays happily consume 3-D references if the shapes match.

What are the limitations?

If your data lacks any Monday entries, the formula returns #CALC!. Wrap with IFERROR. Large datasets (millions of rows) may exceed worksheet limits; switch to Power Pivot in that scenario.

How do I handle errors?

Chain IFERROR or IFNA around the outermost LET. For example:

=IFERROR( your_formula , "No Monday tides found")

Also validate input data types with ISTEXT or ISNUMBER before aggregation.

Does this work in older Excel versions?

The modern dynamic-array version requires 365 or 2021. For 2016 and earlier, use the array formula variant given in the Best Approach section and confirm with Ctrl + Shift + Enter.

What about performance with large datasets?

Dynamic arrays recalculate far faster than manual filters because they leverage multi-threaded calculation. Keep ranges to necessary rows, turn off automatic calculation during bulk imports, and disable “Enable iterative calculation” unless you need it.

Conclusion

Knowing how to look up the lowest Monday tide combines several core Excel skills—date handling, conditional filtering, aggregation, and dynamic arrays—into one practical technique. Mastering it not only saves time and reduces operational risk for maritime tasks, it also equips you to tackle similar weekday-specific analyses in finance, HR, and manufacturing. Practice the examples, choose the method that fits your Excel version, and soon you will integrate this pattern into dashboards, automated reports, and safety checklists with confidence. Keep exploring, and let each tide analysis broaden your mastery of Excel’s powerful formula language.